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# Two types of earthworm, one black and one red-brown, inhabit

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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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31 Aug 2013, 00:30
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pqhai wrote:
pjaseem wrote:

In the Type 2 , According to you the Assumption is : No other factors caused the reduction in B.

But in Answer C we are just talking about how the climate had no caused the reduction.It may be that water was scarce and the Red ones needed more moisture compared to the blanck ones , hence they died enmasse.How can C strenghten the argument then?

Hi pjaseem

C says the climate is favorable for RED worm population. So the RED worm population should increase or at least as much as in the past (the RED worm population did NOT decrease as you said).
For example:
In the past: RED worm population was 100, Black worm population was 20 (1/5 of 100)
After new manufacturing: RED worm population was 150, black worm population was 150 too.
==> Clearly the new factor "the blackening of the woods" should play important role to support the Black worm population. Hence, C strengthens the conclusion.

Note: Strengthen does not mean support 100%. Even the answer supports 1%, it DOES strengthen the conclusion.

Hope it helps.

Sorry for the late reply.You are right about that.I think C wont be the answer if the question was "logically justify" in which case the answer has to provide 100% support
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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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27 Oct 2013, 20:04
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TGC wrote:
Two types of earthworm, one black and one red-brown, inhabit the woods near the town of Millerton. Because the red-brown worm's coloring affords it better camouflage from predatory birds, its population in 1980 was approximately five times that of the black worm. In 1990, a factory was built in Millerton and emissions from the factory blackened much of the woods. The population of black earthworms is now almost equal to that of the red-brown earthworm, a result, say local ecologists, solely stemming from the blackening of the woods.

Which of the following, if true, would most strengthen the conclusion of the local ecologists?

A]The number of red-brown earthworms in the Millerton woods has steadily dropped since the factory began operations.
B]The birds that prey on earthworms prefer black worms to red-brown worms.
C]Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm.
D] The average life span of the earthworms has remained the same since the factory began operations.
E]Since the factory took steps to reduce emissions six months ago, there has been a slight increase in the earthworm population.

source:Veritas prep

What is wrong with option
[Reveal] Spoiler:
(A).
As we have to prove that after factory operations the red ones were more visible than the black ones ,so this should provide support to "Black = Red solely stemming from the" blackening of the woods."
Knowing that factory operations was the cause of " blackening of the woods."

A great answer from pqhai to a great question. I would only add my few points in it.

IMO, this question though worded as strengthen question is actually testing whether we can figure out the defender assumptions the author has made while drawing the conclusion.

In general there are two types of assumptions.

1) Supporter Assumption :- This types of assumptions are common and plays traditional linking role - connecting the pieces of the arguments to one another by usually providing new information and filling out the gap between premises and conclusion.

2) Defender Assumptions :- These are of altogether different type. They protect the argument by eliminating the ideas that could weaken the argument.

Consider the following argument :- People who read a lot are more intelligent than other people. Thus, reading must cause a person to be intelligent.

Following are some of the statements that could attack the above conclusion
1) Regular exercise causes a person to be intelligent.
2) A high-protein diet causes a person to be intelligent.

So to hold the conclusion true, author assumes that the above scenarios do not exist.
1) Regular exercise does not cause a person to be intelligent
2) A high-protein diet does not cause a person to be intelligent.

So we can infer that defender assumptions are basically eliminates the possibility of any alternate explanation for the given effect.

Back to the question...........

Premise 1 :- During 1980s population of red-worms was approx 5 times that of black worms, because the coloring of red-worms was matching with the surrounding environment (esp tree's woods) and protecting them from predators.

Premise 2 :- During 1990s emissions from newly built factory blackened much of the woods. This change took back the color match advantage that red-worms were getting till date.

Premise 3 :- At the same time population of red worms became almost equal to that of black worms.

Conclusion :- The decrease in the population of red-worms is due to the blackening of the woods.

Here the ecologist attributed the decrease of red-worms' population solely to the blackening of the woods caused by emission. So while drawing this conclusion he must have assumed that any alternate explanation for the decrease in population of red-worms does not exists.

Since we are told to strengthen the ecologist's conclusion, our job is to refute the possibility of any alternate explanation.

Choice A :- INCORRECT. Shell Game Answer. This choice merely restates the premise. It does not add any new information that would affect the conclusion.

Choice B :- INCORRECT. This choice does not make any sense with the conclusion. If the predators had preferred black worms over red-worms then the population of red-worms certainly would have not dropped substantially.

Choice C :- CORRECT. Changed environmental condition could also be the cause of decrease in red-worms' population. THIS CHOICE eliminates the possibility of existence of such scenario and makes the ecologist's conclusion valid.

Choice D :- INCORRECT. irrelevant

Choice E :- INCORRECT. irrelevant.
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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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03 Dec 2013, 04:41
pqhai wrote:
Tough question. This is an example of the combination of CAUSES & EFFECTS and COMPARISON.

MAIN IDEA:

Before event A, X is less than Y.
After event A, X and Y are equal
Conclusion: Event A causes X increase.

Assumption: Y does not decrease itself or there's NO factors make Y decrease.

Negation to confirm: If Y decreases itself ==> cannot say Event A causes X increase (because X is the same, only Y decreases to make two variables are equal).

(A) The number of red-brown earthworms in the Millerton woods has steadily dropped since the factory began operations.
Wrong. Actually A weakens the conclusionn not strengthen because it says red-brown earthworms decreased by itself.

(B) The birds that prey on earthworms prefer black worms to red-brown worms.
Wrong.. Shell game. Birds prefer black worms ==> why birds prefer black worms, because birds can see black worms ==> if they cannot see black worms, how can they prefer them? ==> we can infer that the blackening of the woods does not play any role to help black worms.

(C) Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm.
Correct. C definitely says the number of red worms does not decrease, even red worms increased their population more than in 1980 ==> However, the number of black worms equals to that of red worms ==> the the blackening of the woods must play an important role to help black worms.

(D) The average life span of the earthworms has remained the same since the factory began operations.
Wrong.. D only says the AVERAGE life span of earthworms unchanged. Let analyze an example.

Before: The number of Red earthworms is five times that of Black earthworms. (100 vs 20)
There are 20 Black earthworms (BW), their life span is 11 days
There are 100 Red earthworms (RW), their life span is 5 days
Average life span = [20*11 + 100*5] / 120 = 6 days

After. The number of Red earthworms equals that of Black earthworms. (20 vs 20)
There are 20 BW, their life span is 11 days
There are 20 RW, their life span is 1 day (Let say, Emission from factory affected only RW, leading to RW reduced both its number and its life span. Blackening did not affect BW at all)
Average life span = [20*11 + 20*1] /40 = 6 days

(E) Since the factory took steps to reduce emissions six months ago, there has been a slight increase in the earthworm population.
Wrong.. Apposite answer. Factory reduces emission ==> woods are less blacken ==> But the number of earthworm increase ==> the blackening of the woods does not play any role.

Hope it helps.

Hi pqhai, thanks for the explanation. I got your point. However, I still don't understand why you mentioned that the conclusion was Event A causes X increase.

The argument says: The population of black earthworms is now almost equal to that of the red-brown earthworm, a result, say local ecologists, solely stemming from the blackening of the woods.

It was not stated that X increased. The only thing we can observe is that the number of 2 species is equal. It can be: "Y decreased due to the change of color of the woods (they are blackened). Since the woods are no longer favorable for the camouflage of Y, its number have definitely reduced. Thus, Event A causes Y decrease." In this case, A can be right, so does D, as in the example that you mentioned.

Do I have any mistakes?

Thank you for answering my question.
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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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01 May 2014, 06:50
Thanks, pqhai for those multiple informative posts.

+1 Kudos.
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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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01 May 2014, 09:52
Great explanation by Narenn. Option A - Shell Game Answer. It doesn't add any new information. It's merely restating the premise.
It's not a Weakener in my opinion.
+1 Kudos - to the question and the explanation.
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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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15 May 2014, 13:06
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Expert's post
TGC wrote:
Two types of earthworm, one black and one red-brown, inhabit the woods near the town of Millerton. Because the red-brown worm's coloring affords it better camouflage from predatory birds, its population in 1980 was approximately five times that of the black worm. In 1990, a factory was built in Millerton and emissions from the factory blackened much of the woods. The population of black earthworms is now almost equal to that of the red-brown earthworm, a result, say local ecologists, solely stemming from the blackening of the woods.

Which of the following, if true, would most strengthen the conclusion of the local ecologists?

A] The number of red-brown earthworms in the Millerton woods has steadily dropped since the factory began operations.
B] The birds that prey on earthworms prefer black worms to red-brown worms.
C] Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm.
D] The average life span of the earthworms has remained the same since the factory began operations.
E] Since the factory took steps to reduce emissions six months ago, there has been a slight increase in the earthworm population.

kinjiGC wrote:
Hi Mike,
Can you please explain why Option B) is wrong here :
Thanks,
Kinjal

Dear Kinjal,
I'm happy to respond. I will note that Marcab, pqhai, and Narenn have all made excellent comments on this question.

Conclusion: "The population of black earthworms is now almost equal to that of the red-brown earthworm, a result, say local ecologists, SOLELY stemming from the blackening of the woods."
That highlighted word, "solely," is a hugely important word in this conclusion. We are trying to claim that the two type of earthworms now have close to equal population because of the blackening of the woods AND because of absolutely nothing else. That's the conclusion we are trying to support.

Let's look at (B):
(B) The birds that prey on earthworms prefer black worms to red-brown worms.
First of all, if the birds are eating the black worms, they must be seeing them, which means that the blackening isn't really doing much to protect them. Moreover, we have here one predator, the birds, applying selective pressure on the black earthworms, pushing there population down. In order for the two types of earthworms to be more-or-less equal in population, either something else must be accounting for more black earthworms, or something else must be accounting for fewer red-brown earthworms. Something else. Is it the blackening of the woods? Other predators? Environmental conditions? Maybe. We don't know. This is, at best, a pretty lame strengthener, because the most we can conclude is that something is hurting the red-brown earthworms or helping the black earthworms, but we don't know what.

Does all this make sense?
Mike
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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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02 Jun 2014, 00:48
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Expert's post
TGC wrote:
Two types of earthworm, one black and one red-brown, inhabit the woods near the town of Millerton. Because the red-brown worm's coloring affords it better camouflage from predatory birds, its population in 1980 was approximately five times that of the black worm. In 1990, a factory was built in Millerton and emissions from the factory blackened much of the woods. The population of black earthworms is now almost equal to that of the red-brown earthworm, a result, say local ecologists, solely stemming from the blackening of the woods.

Which of the following, if true, would most strengthen the conclusion of the local ecologists?

A]The number of red-brown earthworms in the Millerton woods has steadily dropped since the factory began operations.
B]The birds that prey on earthworms prefer black worms to red-brown worms.
C]Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm.
D] The average life span of the earthworms has remained the same since the factory began operations.
E]Since the factory took steps to reduce emissions six months ago, there has been a slight increase in the earthworm population.

source:Veritas prep

What is wrong with option
[Reveal] Spoiler:
(A).
As we have to prove that after factory operations the red ones were more visible than the black ones ,so this should provide support to "Black = Red solely stemming from the" blackening of the woods."
Knowing that factory operations was the cause of " blackening of the woods."

As requested, here are my thoughts on the question:

Premises:
Two types of worms - red and black
Red has better camouflage from predatory birds and hence its population was 5 times that of black
Factory has blackened woods and now population of both is same (implying that black has better camouflage than before now due to blackening of woods)

Conclusion:
Blackening of woods (and hence better camouflage from predatory birds) is solely responsible for equalization of the two populations.

We need to strengthen the conclusion. Note that there is no doubt that blackening of the woods is responsible for equalization of populations. The question is whether it is solely responsible.

Option (C) tells you that another factor that could have had an effect (i.e. climate) is not responsible. This strengthens the conclusion that better camouflage is SOLELY responsible. It doesn't prove the conclusion beyond doubt since there could still be another factor that could be responsible but it does discard one of the other factors. So it does improve the probability of the conclusion being true.

Note that option (B) strengthens that 'blackening of the woods is responsible for equalization of population' but it DOES NOT strengthen that 'blackening of the woods is SOLELY responsible for equalization'. - That is where the difference lies between (B) and (C)
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Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Intern Joined: 04 Jun 2012 Posts: 47 Location: India Concentration: Finance, General Management GPA: 3.1 WE: Project Management (Energy and Utilities) Followers: 0 Kudos [?]: 20 [0], given: 20 Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink] ### Show Tags 26 Jul 2014, 08:11 VeritasPrepKarishma wrote: TGC wrote: Two types of earthworm, one black and one red-brown, inhabit the woods near the town of Millerton. Because the red-brown worm's coloring affords it better camouflage from predatory birds, its population in 1980 was approximately five times that of the black worm. In 1990, a factory was built in Millerton and emissions from the factory blackened much of the woods. The population of black earthworms is now almost equal to that of the red-brown earthworm, a result, say local ecologists, solely stemming from the blackening of the woods. Which of the following, if true, would most strengthen the conclusion of the local ecologists? A]The number of red-brown earthworms in the Millerton woods has steadily dropped since the factory began operations. B]The birds that prey on earthworms prefer black worms to red-brown worms. C]Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm. D] The average life span of the earthworms has remained the same since the factory began operations. E]Since the factory took steps to reduce emissions six months ago, there has been a slight increase in the earthworm population. source:Veritas prep What is wrong with option [Reveal] Spoiler: (A). As we have to prove that after factory operations the red ones were more visible than the black ones ,so this should provide support to "Black = Red solely stemming from the" blackening of the woods." Knowing that factory operations was the cause of " blackening of the woods." As requested, here are my thoughts on the question: Premises: Two types of worms - red and black Red has better camouflage from predatory birds and hence its population was 5 times that of black Factory has blackened woods and now population of both is same (implying that black has better camouflage than before now due to blackening of woods) Conclusion: Blackening of woods (and hence better camouflage from predatory birds) is solely responsible for equalization of the two populations. We need to strengthen the conclusion. Note that there is no doubt that blackening of the woods is responsible for equalization of populations. The question is whether it is solely responsible. Option (C) tells you that another factor that could have had an effect (i.e. climate) is not responsible. This strengthens the conclusion that better camouflage is SOLELY responsible. It doesn't prove the conclusion beyond doubt since there could still be another factor that could be responsible but it does discard one of the other factors. So it does improve the probability of the conclusion being true. Note that option (B) strengthens that 'blackening of the woods is responsible for equalization of population' but it DOES NOT strengthen that 'blackening of the woods is SOLELY responsible for equalization'. - That is where the difference lies between (B) and (C) Hi Karishma, I got stuck at option A. My reasoning was that because the factory began operations, more woods were turning black, as a result black earth-worms became better at camouflaging. This option indicates that the red earthworms are no longer able to camouflage and hence their population is decreasing. I believe that this option is also strengthening the conclusion. On the test I marked A and got it wrong . Could you please explain why A is wrong? _________________ KUDOS if you find it good!! Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 7118 Location: Pune, India Followers: 2128 Kudos [?]: 13625 [0], given: 222 Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink] ### Show Tags 27 Jul 2014, 20:39 akshaygaur wrote: VeritasPrepKarishma wrote: TGC wrote: Two types of earthworm, one black and one red-brown, inhabit the woods near the town of Millerton. Because the red-brown worm's coloring affords it better camouflage from predatory birds, its population in 1980 was approximately five times that of the black worm. In 1990, a factory was built in Millerton and emissions from the factory blackened much of the woods. The population of black earthworms is now almost equal to that of the red-brown earthworm, a result, say local ecologists, solely stemming from the blackening of the woods. Which of the following, if true, would most strengthen the conclusion of the local ecologists? A]The number of red-brown earthworms in the Millerton woods has steadily dropped since the factory began operations. B]The birds that prey on earthworms prefer black worms to red-brown worms. C]Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm. D] The average life span of the earthworms has remained the same since the factory began operations. E]Since the factory took steps to reduce emissions six months ago, there has been a slight increase in the earthworm population. source:Veritas prep What is wrong with option [Reveal] Spoiler: (A). As we have to prove that after factory operations the red ones were more visible than the black ones ,so this should provide support to "Black = Red solely stemming from the" blackening of the woods." Knowing that factory operations was the cause of " blackening of the woods." As requested, here are my thoughts on the question: Premises: Two types of worms - red and black Red has better camouflage from predatory birds and hence its population was 5 times that of black Factory has blackened woods and now population of both is same (implying that black has better camouflage than before now due to blackening of woods) Conclusion: Blackening of woods (and hence better camouflage from predatory birds) is solely responsible for equalization of the two populations. We need to strengthen the conclusion. Note that there is no doubt that blackening of the woods is responsible for equalization of populations. The question is whether it is solely responsible. Option (C) tells you that another factor that could have had an effect (i.e. climate) is not responsible. This strengthens the conclusion that better camouflage is SOLELY responsible. It doesn't prove the conclusion beyond doubt since there could still be another factor that could be responsible but it does discard one of the other factors. So it does improve the probability of the conclusion being true. Note that option (B) strengthens that 'blackening of the woods is responsible for equalization of population' but it DOES NOT strengthen that 'blackening of the woods is SOLELY responsible for equalization'. - That is where the difference lies between (B) and (C) Hi Karishma, I got stuck at option A. My reasoning was that because the factory began operations, more woods were turning black, as a result black earth-worms became better at camouflaging. This option indicates that the red earthworms are no longer able to camouflage and hence their population is decreasing. I believe that this option is also strengthening the conclusion. On the test I marked A and got it wrong . Could you please explain why A is wrong? The conclusion is that ONLY blackening of the woods caused the numbers to equalize (either black worms were better able to hide or red worms were not able to hide or both happened). We need to look for the option that strengthens that there is NO OTHER reason. Option (A) only tells you what the argument does anyway - red worms population is decreasing due to the blackening of the woods. It doesn't strengthen the claim that ONLY blackening of the woods is responsible. _________________ Karishma Veritas Prep | GMAT Instructor My Blog Get started with Veritas Prep GMAT On Demand for$199

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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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02 Nov 2014, 05:16
pqhai wrote:
Tough question. This is an example of the combination of CAUSES & EFFECTS and COMPARISON.

MAIN IDEA:

Before event A, X is less than Y.
After event A, X and Y are equal
Conclusion: Event A causes X increase.

Assumption: Y does not decrease itself or there's NO factors make Y decrease.

Negation to confirm: If Y decreases itself ==> cannot say Event A causes X increase (because X is the same, only Y decreases to make two variables are equal).

(A) The number of red-brown earthworms in the Millerton woods has steadily dropped since the factory began operations.
Wrong. Actually A weakens the conclusionn not strengthen because it says red-brown earthworms decreased by itself.

(B) The birds that prey on earthworms prefer black worms to red-brown worms.
Wrong.. Shell game. Birds prefer black worms ==> why birds prefer black worms, because birds can see black worms ==> if they cannot see black worms, how can they prefer them? ==> we can infer that the blackening of the woods does not play any role to help black worms.

(C) Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm.
Correct. C definitely says the number of red worms does not decrease, even red worms increased their population more than in 1980 ==> However, the number of black worms equals to that of red worms ==> the the blackening of the woods must play an important role to help black worms.

(D) The average life span of the earthworms has remained the same since the factory began operations.
Wrong.. D only says the AVERAGE life span of earthworms unchanged. Let analyze an example.

Before: The number of Red earthworms is five times that of Black earthworms. (100 vs 20)
There are 20 Black earthworms (BW), their life span is 11 days
There are 100 Red earthworms (RW), their life span is 5 days
Average life span = [20*11 + 100*5] / 120 = 6 days

After. The number of Red earthworms equals that of Black earthworms. (20 vs 20)
There are 20 BW, their life span is 11 days
There are 20 RW, their life span is 1 day (Let say, Emission from factory affected only RW, leading to RW reduced both its number and its life span. Blackening did not affect BW at all)
Average life span = [20*11 + 20*1] /40 = 6 days

(E) Since the factory took steps to reduce emissions six months ago, there has been a slight increase in the earthworm population.
Wrong.. Apposite answer. Factory reduces emission ==> woods are less blacken ==> But the number of earthworm increase ==> the blackening of the woods does not play any role.

Hope it helps.

I still disagree with the explanation given for option B
Obviously the bird needs to survive so if it cannot find the black worm it will eat the red worm, bringing down the red worm's population. The camouflage provided by blackening of woods is certainly helping the black worm to survive and increase in number. So two effects : decrease in red worm and increase in black worm definitely helps more than option C. I still think B should be the answer
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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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02 Nov 2014, 05:21
VeritasPrepKarishma wrote:
TGC wrote:
Two types of earthworm, one black and one red-brown, inhabit the woods near the town of Millerton. Because the red-brown worm's coloring affords it better camouflage from predatory birds, its population in 1980 was approximately five times that of the black worm. In 1990, a factory was built in Millerton and emissions from the factory blackened much of the woods. The population of black earthworms is now almost equal to that of the red-brown earthworm, a result, say local ecologists, solely stemming from the blackening of the woods.

Which of the following, if true, would most strengthen the conclusion of the local ecologists?

A]The number of red-brown earthworms in the Millerton woods has steadily dropped since the factory began operations.
B]The birds that prey on earthworms prefer black worms to red-brown worms.
C]Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm.
D] The average life span of the earthworms has remained the same since the factory began operations.
E]Since the factory took steps to reduce emissions six months ago, there has been a slight increase in the earthworm population.

source:Veritas prep

What is wrong with option
[Reveal] Spoiler:
(A).
As we have to prove that after factory operations the red ones were more visible than the black ones ,so this should provide support to "Black = Red solely stemming from the" blackening of the woods."
Knowing that factory operations was the cause of " blackening of the woods."

As requested, here are my thoughts on the question:

Premises:
Two types of worms - red and black
Red has better camouflage from predatory birds and hence its population was 5 times that of black
Factory has blackened woods and now population of both is same (implying that black has better camouflage than before now due to blackening of woods)

Conclusion:
Blackening of woods (and hence better camouflage from predatory birds) is solely responsible for equalization of the two populations.

We need to strengthen the conclusion. Note that there is no doubt that blackening of the woods is responsible for equalization of populations. The question is whether it is solely responsible.

Option (C) tells you that another factor that could have had an effect (i.e. climate) is not responsible. This strengthens the conclusion that better camouflage is SOLELY responsible. It doesn't prove the conclusion beyond doubt since there could still be another factor that could be responsible but it does discard one of the other factors. So it does improve the probability of the conclusion being true.

Note that option (B) strengthens that 'blackening of the woods is responsible for equalization of population' but it DOES NOT strengthen that 'blackening of the woods is SOLELY responsible for equalization'. - That is where the difference lies between (B) and (C)

Now this does make more sense now !
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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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06 Nov 2014, 03:37
pqhai wrote:
Tough question. This is an example of the combination of CAUSES & EFFECTS and COMPARISON.

MAIN IDEA:

Before event A, X is less than Y.
After event A, X and Y are equal
Conclusion: Event A causes X increase.

Assumption: Y does not decrease itself or there's NO factors make Y decrease.

Negation to confirm: If Y decreases itself ==> cannot say Event A causes X increase (because X is the same, only Y decreases to make two variables are equal).

(A) The number of red-brown earthworms in the Millerton woods has steadily dropped since the factory began operations.
Wrong. Actually A weakens the conclusionn not strengthen because it says red-brown earthworms decreased by itself.

(B) The birds that prey on earthworms prefer black worms to red-brown worms.
Wrong.. Shell game. Birds prefer black worms ==> why birds prefer black worms, because birds can see black worms ==> if they cannot see black worms, how can they prefer them? ==> we can infer that the blackening of the woods does not play any role to help black worms.

(C) Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm.
Correct. C definitely says the number of red worms does not decrease, even red worms increased their population more than in 1980 ==> However, the number of black worms equals to that of red worms ==> the the blackening of the woods must play an important role to help black worms.

(D) The average life span of the earthworms has remained the same since the factory began operations.
Wrong.. D only says the AVERAGE life span of earthworms unchanged. Let analyze an example.

Before: The number of Red earthworms is five times that of Black earthworms. (100 vs 20)
There are 20 Black earthworms (BW), their life span is 11 days
There are 100 Red earthworms (RW), their life span is 5 days
Average life span = [20*11 + 100*5] / 120 = 6 days

After. The number of Red earthworms equals that of Black earthworms. (20 vs 20)
There are 20 BW, their life span is 11 days
There are 20 RW, their life span is 1 day (Let say, Emission from factory affected only RW, leading to RW reduced both its number and its life span. Blackening did not affect BW at all)
Average life span = [20*11 + 20*1] /40 = 6 days

(E) Since the factory took steps to reduce emissions six months ago, there has been a slight increase in the earthworm population.
Wrong.. Apposite answer. Factory reduces emission ==> woods are less blacken ==> But the number of earthworm increase ==> the blackening of the woods does not play any role.

Hope it helps.

pqhai

You seemed to have opened a can of worms ( just kidding with that idiom)

BW= Black Worms
The only point of contention for me is that you have assumed A and drawn a conclusion based on it.

I am implying that there is NO evidence in the passage to suggest that BW population increased. The likely hood of RW population to decrease is as much as the increase of BW.

Blackening of woods has been given as the reason for RW=BW but there is no specific means to assume that BW increased and NOT RW decreased.

Blackening of woods may have contributed to the decrease in RW.
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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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29 Apr 2015, 12:09
pqhai wrote:
BangOn wrote:
I understand why C is better but can you help me explain why D is bad?
Another POV can be that since life spans have not changed, so blackening is doing the damage. This is like other things remaining same, blackening is an issue.

VERY GOOD question!
Sorry for not explaining D. Here is it.

D only says the AVERAGE life span of earthworms unchanged. Let analyze an example.

Before: The number of Red earthworms is five times that of Black earthworms. (100 vs 20)

There are 20 Black earthworms (BW), their life span is 11 days
There are 100 Red earthworms (RW), their life span is 5 days
Average life span = [20*11 + 100*5] / 120 = 6 days

After. The number of Red earthworms equals that of Black earthworms. (20 vs 20)

There are 20 BW, their life span is 11 days
There are 20 RW, their life span is 1 day (Let say, Emission from factory affected only RW, leading to RW reduced both its number and its life span. Blackening did not affect BW at all)
Average life span = [20*11 + 20*1] /40 = 6 days

Now you understand why AVERAGE life span does not help anything?

Hope it helps you.

Regards.

I have the same doubt for D) but I didn't understand your explanation for it! Can you please elaborate
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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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30 Apr 2015, 08:23
A]The number of red-brown earthworms in the Millerton woods has steadily dropped since the factory began operations.
This shows correlation, not causation.

B]The birds that prey on earthworms prefer black worms to red-brown worms.
This proves that the birds would eat more black worms if given the choice. If the forest is darkened, the camouflage would be much better for the black earthworms who are indeed black. The birds thus has to choose red-brown worms.

C]Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm.
This is also a good choice. If red-brown has had a better future outlook in survival terms, then it would be odd that they have been outrun by the black worm, unless the red brown died from other reasons, such as the birds. Still, this is mainly correlation and no causation.

D] The average life span of the earthworms has remained the same since the factory began operations.
If the average life span is the same, this would imply that the worms are not dying earlier than usually. This would not strengthen the argument, but would imply that the difference in inflow of new worms between RB and B has changed in favor for B. Not helping the conclusion which blamed the birds.

E]Since the factory took steps to reduce emissions six months ago, there has been a slight increase in the earthworm population.
Again, correlation, not causation.
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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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18 Oct 2015, 00:46
Anu26 wrote:
pqhai wrote:
BangOn wrote:
I understand why C is better but can you help me explain why D is bad?
Another POV can be that since life spans have not changed, so blackening is doing the damage. This is like other things remaining same, blackening is an issue.

VERY GOOD question!
Sorry for not explaining D. Here is it.

D only says the AVERAGE life span of earthworms unchanged. Let analyze an example.

Before: The number of Red earthworms is five times that of Black earthworms. (100 vs 20)

There are 20 Black earthworms (BW), their life span is 11 days
There are 100 Red earthworms (RW), their life span is 5 days
Average life span = [20*11 + 100*5] / 120 = 6 days

After. The number of Red earthworms equals that of Black earthworms. (20 vs 20)

There are 20 BW, their life span is 11 days
There are 20 RW, their life span is 1 day (Let say, Emission from factory affected only RW, leading to RW reduced both its number and its life span. Blackening did not affect BW at all)
Average life span = [20*11 + 20*1] /40 = 6 days

Now you understand why AVERAGE life span does not help anything?

Hope it helps you.

Regards.

I have the same doubt for D) but I didn't understand your explanation for it! Can you please elaborate

Conclusion - Blackening of woods , therefore better camouflage from predatory birds for Black earthworms.
This is solely responsible for equalization of the population of red brown and black earthworms.

As per explanation by pqhai for Option D , the emission from factory affected only Red brown earthworms ,leading to
reduction in number and life span of RB earthworms .
It doesn't support our conclusion the blackening lead to better camouflage --> increase in black earthworm number.

IMO D is irrelevant as it does not strengthen the conclusion .

Option C- It tells that an alternate cause ( climate conditions ) which could have had an effect is not responsible. Thus , it strengthens the conclusion that
camouflage is solely responsible .

This is an incredible question . Thanks to all the amazing explanations !! Cheers
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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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15 Apr 2016, 03:22
pqhai wrote:
Tough question. This is an example of the combination of CAUSES & EFFECTS and COMPARISON.

MAIN IDEA:

Before event A, X is less than Y.
After event A, X and Y are equal
Conclusion: Event A causes X increase.

Assumption: Y does not decrease itself or there's NO factors make Y decrease.

Negation to confirm: If Y decreases itself ==> cannot say Event A causes X increase (because X is the same, only Y decreases to make two variables are equal).

(A) The number of red-brown earthworms in the Millerton woods has steadily dropped since the factory began operations.
Wrong. Actually A weakens the conclusionn not strengthen because it says red-brown earthworms decreased by itself.

.

It can also represent the fact that ->Because the red-brown worm's coloring affords it better camouflage from predatory birds -> now woods are blackening --> so red brown worm coloring effect not helping so much --> there no should decrease if blackening of woods.
That's what statement A is. It's strengthening if your line of thinking goes like this.
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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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19 Apr 2016, 03:11
Logic of GMAC is:
Before Event X: A was LESS than B.
After Event X: A equals B. OR A is larger than B.
Conclusion: X must cause A increase.
Assumption is: No factors make B decrease
I think that after the event X,A did not increase but B decreased due to predation so how can the conclusion be X must cause A increase ( although A might have increased through reproduction) At some point both A and B became equal

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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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19 Apr 2016, 10:03
shivakumarmakka wrote:
Logic of GMAC is:
Before Event X: A was LESS than B.
After Event X: A equals B. OR A is larger than B.
Conclusion: X must cause A increase.
Assumption is: No factors make B decrease
I think that after the event X,A did not increase but B decreased due to predation so how can the conclusion be X must cause A increase ( although A might have increased through reproduction) At some point both A and B became equal

Dear shivakumarmakka
I'm happy to respond.

Let's say
A = black earthworm population
B = red-brown earthworm population

Before event, A < B. Technically, A = 5B, approximately.

BUT, be careful. After event, it does not say A > B or even A = B. The exact wording is
"The population of black earthworms is now almost equal to that of the red-brown earthworm ..."
Technically, A is still less than B: now, not five times less, but just slightly less. It's misreading the question to say that A is greater than B.

Also, notice that nowhere in the question itself does it suggest that the black worms have increased. The prompt argument doesn't even contain the word "increase." In fact, the question is about B decreasing. The factory has blackened the woods, and presumably the black worms are now well camouflaged, but the red-brown worms are easier to spot, so the birds snack on these. We don't know whether the black worms have stayed the same or increases, but through some combination of B dropping and A staying the same or rising, A has almost caught up to B.

My friend, you have to remember that GMAT CR is as precise as mathematics. You have to look at every single word incredibly carefully. It appears you got the gist of the argument and concluded in your own mind that it was saying things different from what it actually says. Be very careful. As in any argument in the business world, the fine print matters, and many business deals that look great on the surface turn out to be nightmares when you examine the details.

Does all this make sense?
Mike
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Re: Two types of earthworm, one black and one red-brown, inhabit [#permalink]

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20 Apr 2016, 14:57
Thank You Mr.Mike, that was helpful indeed...sure it does:-D
Re: Two types of earthworm, one black and one red-brown, inhabit   [#permalink] 20 Apr 2016, 14:57

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