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

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).

CONSIDER EACH ANSWER:

(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.

Let us first look at answer choice C and try to understand how does it strengthens the conclusion of ecologists.
Conclusion:The result says that the population of black earthworms is now almost equal to that of the red-brown earthworm, solely stemming from the blackening of the woods.
C says that "Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm".
Since climate favours the survival of red-brown worm than to the black worm, then chances are quite high that the # of red-brown worms must be very high if compared to that of black worm. Even then, the # of black worms is equal to # of red-brown worms, then something must have been wrong and some factor has played a role. What C does is that it eliminates any other chance and brings to us that the only way # of red-brown worms is equal to # of black worms is due to the blackening of woods.

Regards.

Dear umeshpatil:
Thank you so much. I really appreciate!!! .

I guess you have practiced a lot for CR. However, from my own experience, there are two very popular types of cause and effect OR C&F with comparison in GMAT.

TYPE 1: Cause and Effect between A & B.
Logic of GMAC is:
If A and B both occur.
Conclusion: A causes B happen.
Assumption is: B does not cause A happen.

For example: People who have high GMAT score test always wear nearsighted glasses.
Conclusion: wearing nearsighted glasses makes people get high GMAT score.
Assumption: Having high GMAT score does not make people have eyes problem, leading to wearing nearsighted glasses.

TYPE 2: Mix of Cause & Effect AND comparison.
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

For example: Like the question we discuss in this thread.

You will see the two question types above present in CR over and over again. When you see two things both happen, and then conclude A causes B. Or when you see before event A, after event X ==> think about cause and effect. You can pick the correct answer in your mind, even when you haven't seen the answers.

I Hope it helps you a little bit.

Regards.

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.[/quote]

I think you are confused because you didn't notice the conclusion carefully. Conclusion states that the population of black worms is equal to red-brown worms caused solely stemming from the blackening of the woods.

Two scenarios can happen-
1) Population of black worms increase while population of red-brown worms remains constant.
2) Population of black worms remains constant while population of red-brown worms decreases.
But no where it is mentioned which scenario actually happened. But the main point is that this change in population is entirely caused by blackening of the woods.
So this is basically Cause & Effect reasoning. In order to strengthen such type of reasoning, we have to prove that NO OTHER scenario can cause this relative change in population.

Option C states that climate was more favorable for red-brown worms than black worms. If this is to be taken as true, it refutes scenario 1.
What bout scenario 2.
Population of black worms remains constant while population of red-brown worms decreases. HOW
Even the climate is more favorable for red-brown worms, yet their population decreased because they lost Camouflage advantage as all the surroundings are turned black, they can be easily found & preyed. This proves that it was blackening of the woods that led to this change in relative population

Hope this helps

VERITAS PREP OFFICIAL SOLUTION:

Correct Answer: C

The argument concludes that the change in coloration of the woods has been the sole cause of the change in the earthworm population. To strengthen the conclusion that the coloration was the sole cause of the improved status of the black earthworms, eliminate other possible causes. Choice C says that the change was not about the climate, which favored the red-browns, making it the correct answer. Choice A allows for the possibility that the equal number of worms was because of a drop in the red-browns, not an increase in the blacks, which might weaken the conclusion. Ultimately, though, it doesn't tell us why the red-browns dropped, so it doesn't really have any effect. For answer choice B, what the birds prefer is irrelevant; the argument is about what they actually eat. Answer D provides us no way to distinguish between the two types of worms. While E may be good to know, it does not strengthen the conclusion.
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Actually, IMO, A weakens the argument.
A says that the number of red worms decreased steadily. So it brings up another factor behind the equalling of the number of black and red worms. Chances can be quite high that because of the factory operations, the # of red worms decreased. So where is the factor of "blackening of wood"? Hence it weakens.

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.

Premise states that due to
"Red Color" of wood red -worms were not easily preyed upon,while the blacks were so R = 5 times the B

As stated in RED quoted above .

Industry set -up => Blackening of woods => easy contrast for REDS bad contrast for BLACKS => Drop in numbers of RED which supports the conclusion in green ,this causal relation as I mentioned above supports the conclusion . Doesn't it?

I can say the same thing with B: "The birds that prey on earthworms prefer black worms to red-brown worms." B also suggests, just like C, that despite birds' preference to black worms, they are now equal in number to red brown worms. This can strengthen the conclusion well.

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.

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.

pqhai, +2 kudos for you . Very Nice explanation for this difficult question.

Do you have any generic suggestion to deal with CAUSES & EFFECTS and COMPARISON

Here are my 2 cents...

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?

I've highlighted the pieces separately to draw attention to the role each piece plays independently.

The word result, as we know, signals the conclusion. This can be deciphered even if you are the kind of person that does not read the question first.

If you have read the question before the passage you know that say local ecologists in the passage clearly correlates with the question, telling you exactly where the conclusion lies. You would be able to decipher this even if you missed the word result.

That said, the conclusion follows. Pay attention to the words; they say solely stemming from the blackening of the woods. This means there is no reason why red-brown earthworms would dwindle.

Now you need to find the choice that strengthens this idea, that is, while favoring both types of what could contribute to the rise in black earthworm population?

The former part of the passage already states that black earthworms could not be well disguised and hence often fell prey to predatory birds. So what could be helping their proliferation now? Blackening of the woods. What can strengthen this position? Eliminate the choices that suggest the red-brown earthworms are dwindling.

A) The number of red-brown earthworms in the Millerton woods has steadily dropped since the factory began operations.
This is a weakening idea. Plus it could go contrary to what the passage suggests.

B) The birds that prey on earthworms prefer black worms to red-brown worms.
This may be close, so I would keep it

C) Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm.
This option clearly strengthens the conclusion. Although I would say choice B goes with the tide, it does not to anything to strengthen the conclusion. Now we don't want a choice that simply states the lines in the passage again. This choice states that even though climatic conditions favored the red-brown worms, the black worms were able to proliferate. Why? Because the woods blackened and the birds were not able to prey on the worms.

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.
Irrelevant choices.
_________________

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.

Brief about Defender Assumptions

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.

Conclusion: The emission is the reason causing the drop of red-brown earthworm

A - Repeat information in premise => cannot be STRENGTHEN
B - Focus on Cause - effect relation Emission - Drop of red-brown earthworm. SO this is out of scope
C - Out of scope too
D - Out of scope because mentioning to LIFE Span
E - It means that emission is the cause of red-brown population drop

Ans E

Let the Two types of earthworm, one black and one red-brown, be Bl and RB.

• RB's coloring affords it better camouflage from predatory birds so they survive better.
  (Red camouflaging can be due to leaves that are red since red woods dont exist in general.)
  
• RB population in 1980 was approximately five times that of the black worm. (RB>Bl)
  
• In 1990, a factory was built in Millerton and emissions from the factoryblackened 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(because of factory).

(The Bl population is equal to RB population.....i.e., RB=Bl

either

RB decreased with Bl being const
or
Bl increased with RB being const

to reach this scenario.)

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.
(we have no info whether Bl also decreased rapidly along with RB because of factory pollution even if camouflaging helps them survive a bit. if that is the case then we cant say strengthen the conclusion. we have no info whether it remains constant or increases as well. This supports but not completely)

B. The birds that prey on earthworms prefer black worms to red-brown worms.
This does not explain why Bl population increased wrt RB population.

C. Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm.
(This supports causal assumption missed in A that factory population did not affect RB, so only lack of camouflaging led to reduction of RB population.)

D. The average life span of the earthworms has remained the same since the factory began operations.
This does not explain why Bl population increased wrt RB population.

E. Since the factory took steps to reduce emissions six months ago, there has been a slight increase in the earthworm population.
This does not explain why Bl population increased wrt RB population.

Correct me if I'm wrong.

Causality!
Given: Woods blacken because of factory's emission (cause) => Red-brown worms lost the camouflage advantage and are preyed upon more. (effect)
We need to strengthen this.

General theory regarding causal arguments- Implicit assumptions by the author:
1) Cause happens before the effect
2) Effect does not cause the cause
3) There is no alternate cause for the effect (the stated cause is the only cause)
Any of the above 3 will strengthen the argument.

Answer choice C helps in establishing assumption 3 above. It discards one of the possible alternate causes for decreasing population of red-brown worms- Adverse climate conditions.
C. Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm.

Therefore, this is a clear strengthener.

Hope this makes sense

Any answer choice which says that the population of black earthworm increased because of factory emissions is my answer. "B" suggests that though birds prefer black earthworms to red ones it necessarily does not suggest that these birds ate more black earthworms than reds."E" weakens the ecologists'contention. If factory emissions reduces then the population of black earthworm should have decreased and not increased because these no longer were camouflaged.In "C" climatic conditions did change because of the emissions. So maybe the new climatic conditions were favorable for the Black worms. Hence "C" is the answer.
_________________

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
Can anyone please clarify!?

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