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505-555 (Easy)|   Complete the Passage|                                 
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AjiteshArun
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When mercury-vapor streetlights are used in areas inhabited by insect 08 Sep 2019, 22:02
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varotkorn
Dear VeritasKarishma AjiteshArun jennpt

Is option B. an assumption in the argument?

(B) the bats are unlikely to feed on insects that do not fly

In order for the change in streetlights to have any impact on the bats, the bats need to feed mostly on flying insects. And that is what option B. states. So, I think option B. is an assumption, and hence a strengthener.

Also negated choice B (the bats are likely to feed on insects that do not fly) clearly destroys the conclusion that this change is likely to result in a drop in the population of insect-eating bats in Greenville.


I am very confused here.

Thank you in advance :)
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Hi varotkorn,

Option B is not an assumption. That is, it is not necessary that the bats be unlikely to feed on insects that do not fly. To see why, we should start with the very first sentence:

When mercury-vapor streetlights are used in areas inhabited by insect-eating bats, the bats feed almost exclusively around the lights, because the lights attract flying insects.

What does this statement tell us? It says that:
(a) in areas with IE bats
(b) when MV lights are used
(c) the bats feed almost exclusively around the lights
(d) because the lights attract flying insects

The statement itself does not tell us that the population of bats increases with the introduction of MV lights. It just says that when MV lights are used, the bats switch from their ~normal feeding areas to (almost exclusively) around the lights.

If we take away the MV lights, either by removing them or by replacing them with lights that don't attract flying insects, we'd expect the bats to go back to their normal feeding areas, and eat their normal diet of delicious (flying/non-flying) insects. That is, in the absence of any further information, we would not expect their numbers to drop.

The correct option gives us a reason to think that the population is affected by the introduction of MV lights, but option B does not do that. It just says that the bats are unlikely to feed on non-flying insects. That's fine, but if the bats go back to their old feeding areas/diets, we don't have (again, in the absence of any further information) any reason to think that the population of insect-eating bats will decline.
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When mercury-vapor streetlights are used in areas inhabited by insect 09 Sep 2019, 00:32
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varotkorn
Dear VeritasKarishma AjiteshArun jennpt

Is option B. an assumption in the argument?

(B) the bats are unlikely to feed on insects that do not fly

In order for the change in streetlights to have any impact on the bats, the bats need to feed mostly on flying insects. And that is what option B. states. So, I think option B. is an assumption, and hence a strengthener.

Also negated choice B (the bats are likely to feed on insects that do not fly) clearly destroys the conclusion that this change is likely to result in a drop in the population of insect-eating bats in Greenville.


I am very confused here.

Thank you in advance :)
Hi varotkorn,

Option B is not an assumption. That is, it is not necessary that the bats be unlikely to feed on insects that do not fly. To see why, we should start with the very first sentence:

When mercury-vapor streetlights are used in areas inhabited by insect-eating bats, the bats feed almost exclusively around the lights, because the lights attract flying insects.

What does this statement tell us? It says that:
(a) in areas with IE bats
(b) when MV lights are used
(c) the bats feed almost exclusively around the lights
(d) because the lights attract flying insects

The statement itself does not tell us that the population of bats increases with the introduction of MV lights. It just says that when MV lights are used, the bats switch from their ~normal feeding areas to (almost exclusively) around the lights.

If we take away the MV lights, either by removing them or by replacing them with lights that don't attract flying insects, we'd expect the bats to go back to their normal feeding areas, and eat their normal diet of delicious (flying/non-flying) insects. That is, in the absence of any further information, we would not expect their numbers to drop.

The correct option gives us a reason to think that the population is affected by the introduction of MV lights, but option B does not do that. It just says that the bats are unlikely to feed on non-flying insects. That's fine, but if the bats go back to their old feeding areas/diets, we don't have (again, in the absence of any further information) any reason to think that the population of insect-eating bats will decline.
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Dear AjiteshArun

Thank you for your kind response :))

I have one follow-up question regarding the highlighted portion above.

If the population of bats does NOT increase with the introduction of MV lights, how does the absence of local concentrations of the flying insects on which bats feed reduce the population of bats?

Let's say:
Situation 1 : Before MV lights are installed, there are 100 bats (use A LOT OF energy) + 100 insects
Situation 2 : During MV lights are installed, according to you there are still 100 bats (use LESS energy, but the population is still the same) + 100 insects
Situation 3 : After MV lights are removed/replaced, the situation is the same as Situation 1. So, there should be 100 bats (use A LOT OF energy)+ 100 insects

I do not get the logic if the population of the bats does NOT increase with MV lights, then how removing the MV lights (going back to Situation 1) reduces the bat population?
But if the population of the bats DOES increase with MV lights in Situation 2, then choice D makes sense.

Thank you in advnace !
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When mercury-vapor streetlights are used in areas inhabited by insect 09 Sep 2019, 02:19
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varotkorn
Dear AjiteshArun

Thank you for your kind response :))

I have one follow-up question regarding the highlighted portion above.

If the population of bats does NOT increase with the introduction of MV lights, how does the absence of local concentrations of the flying insects on which bats feed reduce the population of bats?

Let's say:
Situation 1 : Before MV lights are installed, there are 100 bats (use A LOT OF energy) + 100 insects
Situation 2 : During MV lights are installed, according to you there are still 100 bats (use LESS energy, but the population is still the same) + 100 insects
Situation 3 : After MV lights are removed/replaced, the situation is the same as Situation 1. So, there should be 100 bats (use A LOT OF energy)+ 100 insects

I do not get the logic if the population of the bats does NOT increase with MV lights, then how removing the MV lights (going back to Situation 1) reduces the bat population?
But if the population of the bats DOES increase with MV lights in Situation 2, then choice D makes sense.

Thank you in advnace !
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Hi varotkorn,

The correct option introduces additional information that completely changes our view of how good the conclusion is. We cannot ignore it when we're looking at the question as a whole. Our discussion was mostly restricted to the combination of the existing support and option B. Leave those for now though. Look at the structure of the argument:

SUPPORT 1 and SUPPORT 2 lead to CONCLUSION because SUPPORT 3.
or
SUPPORT 1 + SUPPORT 2 + SUPPORT 3 → CONCLUSION

Support 1 and support 2 are already given to us. However, support 3 is something that we need to provide.

If we look at support 1 and support 2 alone, then, in the absence of any further information, we have no reason to think that the population will decrease the way the conclusion asserts it will. Because support 1 and support 2 do not affect the population (but the conclusion is about a decrease in the population), the correct option (the additional information) must have some impact on the population. A quick example:

Support 1: When a company operates a canteen in its campus, employees who cannot bring meals from home have their meals almost exclusively in the company canteen.
Support 2: Company X's canteen is about to close.
Conclusion: This change is going to lead to a drop in the number of employees at company X who cannot bring meals from home because support 3.

Candidates for support 3:
B Employees who cannot bring meals from home are unlikely to eat home-cooked meals.
D In the absence of a canteen, employees who cannot bring meals from home spend so much time and energy finding food that they can't get any work done.

With option B, we're really not looking at a mass exodus of employees. Sure, they can't bring food from home, but they could always go back to doing whatever they were doing when there was no canteen (they could go outside the campus, or they could get something delivered).

D, on the other hand, finally gives us a reason to believe that employees will leave (or be forced to leave) as a result of this change.

Please excuse the choice of example. I've been reading about how tech companies invest in canteens to get more work out of their employees. :)
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When mercury-vapor streetlights are used in areas inhabited by insect 09 Sep 2019, 07:28
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varotkorn
Dear AjiteshArun

Thank you for your kind response :))

I have one follow-up question regarding the highlighted portion above.

If the population of bats does NOT increase with the introduction of MV lights, how does the absence of local concentrations of the flying insects on which bats feed reduce the population of bats?

Let's say:
Situation 1 : Before MV lights are installed, there are 100 bats (use A LOT OF energy) + 100 insects
Situation 2 : During MV lights are installed, according to you there are still 100 bats (use LESS energy, but the population is still the same) + 100 insects
Situation 3 : After MV lights are removed/replaced, the situation is the same as Situation 1. So, there should be 100 bats (use A LOT OF energy)+ 100 insects

I do not get the logic if the population of the bats does NOT increase with MV lights, then how removing the MV lights (going back to Situation 1) reduces the bat population?
But if the population of the bats DOES increase with MV lights in Situation 2, then choice D makes sense.

Thank you in advnace !
Hi varotkorn,

The correct option introduces additional information that completely changes our view of how good the conclusion is. We cannot ignore it when we're looking at the question as a whole. Our discussion was mostly restricted to the combination of the existing support and option B. Leave those for now though. Look at the structure of the argument:

SUPPORT 1 and SUPPORT 2 lead to CONCLUSION because SUPPORT 3.
or
SUPPORT 1 + SUPPORT 2 + SUPPORT 3 → CONCLUSION

Support 1 and support 2 are already given to us. However, support 3 is something that we need to provide.

If we look at support 1 and support 2 alone, then, in the absence of any further information, we have no reason to think that the population will decrease the way the conclusion asserts it will. Because support 1 and support 2 do not affect the population (but the conclusion is about a decrease in the population), the correct option (the additional information) must have some impact on the population. A quick example:

Support 1: When a company operates a canteen in its campus, employees who cannot bring meals from home have their meals almost exclusively in the company canteen.
Support 2: Company X's canteen is about to close.
Conclusion: This change is going to lead to a drop in the number of employees at company X who cannot bring meals from home because support 3.

Candidates for support 3:
B Employees who cannot bring meals from home are unlikely to eat home-cooked meals.
D In the absence of a canteen, employees who cannot bring meals from home spend so much time and energy finding food that they can't get any work done.

With option B, we're really not looking at a mass exodus of employees. Sure, they can't bring food from home, but they could always go back to doing whatever they were doing when there was no canteen (they could go outside the campus, or they could get something delivered).

D, on the other hand, finally gives us a reason to believe that employees will leave (or be forced to leave) as a result of this change.

Please excuse the choice of example. I've been reading about how tech companies invest in canteens to get more work out of their employees. :)
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Dear AjiteshArun

Thank you for your response :)

I am still a little bit confused here. Sorry for the inconvenience caused to you

In choice D., given that the bat population has remained the same, if the bat, although uses more energy, uses the same level of energy before the mercury-vapor streetlights were installed, then the population of the bat would not change right?

Situation 1 : Before MV lights are installed, bats need to eat 150 insects
Situation 2 : During MV lights are installed, bats need to eat only 100 insects
Situation 3 : After MV lights are removed/replaced, bats need to eat 150 insects. So, there should be no problem here.

Thank you for your patience !
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When mercury-vapor streetlights are used in areas inhabited by insect 09 Sep 2019, 10:43
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varotkorn
Dear AjiteshArun

Thank you for your response :)

I am still a little bit confused here. Sorry for the inconvenience caused to you

In choice D., given that the bat population has remained the same, if the bat, although uses more energy, uses the same level of energy before the mercury-vapor streetlights were installed, then the population of the bat would not change right?

Situation 1 : Before MV lights are installed, bats need to eat 150 insects
Situation 2 : During MV lights are installed, bats need to eat only 100 insects
Situation 3 : After MV lights are removed/replaced, bats need to eat 150 insects. So, there should be no problem here.

Thank you for your patience !
Show more
Hi varotkorn,

The mistake you're making is here:
In choice D., given that the bat population has remained the same

When we go with option D, it is entirely possible that the bat population increases when MV lights are introduced, and decreases when MV lights are removed. You may be going from (1) below to (2):

(1) Support 1 and support 2 together don't give us any reason to believe that the population will decrease.
(2) Therefore the population will remain the same.

We can't go from (1) to (2). In fact, (2) is the opposite of the conclusion, which in turn is the very thing that we need to support!

I get the feeling that another question is on its way. Please feel free to ask. :)
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When mercury-vapor streetlights are used in areas inhabited by insect 10 Sep 2019, 02:25
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Which of the following most logically completes the argument below?

When mercury-vapor streetlights are used in areas inhabited by insect-eating bats, the bats feed almost exclusively around the lights, because the lights attract flying insects. In Greenville, the mercury-vapor streetlights are about to be replaced with energy-saving sodium streetlights, which do not attract insects. This change is likely to result in a drop in the population of insect-eating bats in Greenville, since __________.

(A) the bats do not begin to hunt until after sundown

(B) the bats are unlikely to feed on insects that do not fly

(C) the highway department will be able to replace mercury-vapor streetlights with sodium streetlights within a relatively short time and without disrupting the continuity of lighting at the locations of the streetlights

(D) in the absence of local concentrations of the flying insects on which bats feed, the bats expend much more energy on hunting for food, requiring much larger quantities of insects to sustain each bat

(E) bats use echolocation to catch insects and therefore gain no advantage from the fact that insects flying in the vicinity of streetlights are visible at night
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mercury-vapor streetlights - attract insects - so bats feed around lights
energy-saving sodium streetlights - do not attract insects - this will reduce population of bats

What we need to figure out is why no insects concentrated around lights will reduce bat population. We can expect that since insects are not concentrated, they will be flying here and there and bats will need to fly after them for food. We are looking for why the bat population will reduce in this case.

(A) the bats do not begin to hunt until after sundown

Doesn't explain why bat population will reduce if insects are not concentrated together.

(B) the bats are unlikely to feed on insects that do not fly

Doesn't explain why bat population will reduce if insects are not concentrated together.

(C) the highway department will be able to replace mercury-vapor streetlights with sodium streetlights within a relatively short time and without disrupting the continuity of lighting at the locations of the streetlights

Irrelevant. If at all, it says that there should not be any difference.

(D) in the absence of local concentrations of the flying insects on which bats feed, the bats expend much more energy on hunting for food, requiring much larger quantities of insects to sustain each bat

Correct. If insects are not concentrated, bats will need to fly around more to get to them. So they will need more food say from 100 insects a night to 150 insects a night). The insect population may not be able to support current bat population in that case.
So this is certainly a valid reason.

(E) bats use echolocation to catch insects and therefore gain no advantage from the fact that insects flying in the vicinity of streetlights are visible at night

If anything, this says that nothing should change. streetlights shouldn't have any impact.

Answer (D)
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When mercury-vapor streetlights are used in areas inhabited by insect 10 Sep 2019, 03:02
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varotkorn
Dear VeritasKarishma AjiteshArun jennpt

Is option B. an assumption in the argument?

(B) the bats are unlikely to feed on insects that do not fly

In order for the change in streetlights to have any impact on the bats, the bats need to feed mostly on flying insects. And that is what option B. states. So, I think option B. is an assumption, and hence a strengthener.

Also negated choice B (the bats are likely to feed on insects that do not fly) clearly destroys the conclusion that this change is likely to result in a drop in the population of insect-eating bats in Greenville.


I am very confused here.

Thank you in advance :)
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See if this helps too -

(B) is not an assumption. There is no distinction being made between insects that fly and ones that don't.
The distinction is between two different circumstances - insects gathering around the light and insects not gathering around the light.
When we switch to sodium lights, the flying insects don't die or fly away. They just don't concentrate around the light. They are still there and in the same number presumably. So switching to the different light just spreads out the insects. Bats may be eating only flying insects or they may be eating a combination of flying/non flying insects - that doesn't change. Just the ease of availability of flying insects changes.
We don't know how easily insects that don't fly are available. The argument does not talk about them at all. We don't know if there are any non flying insects in the area.
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When mercury-vapor streetlights are used in areas inhabited by insect 10 Sep 2019, 04:48
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varotkorn
Dear AjiteshArun

Thank you for your response :)

I am still a little bit confused here. Sorry for the inconvenience caused to you

In choice D., given that the bat population has remained the same, if the bat, although uses more energy, uses the same level of energy before the mercury-vapor streetlights were installed, then the population of the bat would not change right?

Situation 1 : Before MV lights are installed, bats need to eat 150 insects
Situation 2 : During MV lights are installed, bats need to eat only 100 insects
Situation 3 : After MV lights are removed/replaced, bats need to eat 150 insects. So, there should be no problem here.

Thank you for your patience !
Hi varotkorn,

The mistake you're making is here:
In choice D., given that the bat population has remained the same

When we go with option D, it is entirely possible that the bat population increases when MV lights are introduced, and decreases when MV lights are removed. You may be going from (1) below to (2):

(1) Support 1 and support 2 together don't give us any reason to believe that the population will decrease.
(2) Therefore the population will remain the same.

We can't go from (1) to (2). In fact, (2) is the opposite of the conclusion, which in turn is the very thing that we need to support!

I get the feeling that another question is on its way. Please feel free to ask. :)
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Dear AjiteshArun

Thank you for your quick response and your patience in answering every question.
I really appreciate your help :)

You are right. I have one (last) question. Hope that you are not tired of me yet :p

According to your previous response, you mentioned that The statement itself does not tell us that the population of bats increases with the introduction of MV lights. I completely agree with that.

However, according to your most recent response above, you mentioned that it is entirely possible that the bat population increases when MV lights are introduced, and decreases when MV lights are removed.

I wonder how can you know whether the bat population has remained the same or has increased when the MV lights were installed?

Is that the assumption (that the bat population has increased when the MV lights were installed) I have to make when I mark the correct option D?

I cannot thank you enough for your kindness and helpfulness :)
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When mercury-vapor streetlights are used in areas inhabited by insect 10 Sep 2019, 06:08
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varotkorn
Dear AjiteshArun

Thank you for your quick response and your patience in answering every question.
I really appreciate your help :)

You are right. I have one (last) question. Hope that you are not tired of me yet :p

According to your previous response, you mentioned that The statement itself does not tell us that the population of bats increases with the introduction of MV lights. I completely agree with that.

However, according to your most recent response above, you mentioned that it is entirely possible that the bat population increases when MV lights are introduced, and decreases when MV lights are removed.

I wonder how can you know whether the bat population has remained the same or has increased when the MV lights were installed?

Is that the assumption (that the bat population has increased when the MV lights were installed) I have to make when I mark the correct option D?

I cannot thank you enough for your kindness and helpfulness :)
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Hi varotkorn,

Most CR questions are not really about certainty. That is, we may not know something (for sure), but we still check which option, if true, is most likely to help us answer the question. That's what I mean by "entirely possible": option D "opens the door" for us (to say something about the bat population). I think it's reasonable to say that if D works one way (the population goes down if MV lights are removed), it will work the other way as well (population goes up if MV lights are introduced). Don't let that distract you though! The question is about only a drop in population, and the extra energy requirement that D introduces is a good reason to expect that the population will decrease.

TL;DR: The verbal section is not like quant. We very often have to mark an option that is just the "least bad" among the options presented to us.
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When mercury-vapor streetlights are used in areas inhabited by insect 12 Jun 2020, 07:32
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GMATNinja or anyone who can help.
Option D- We do not know that the insect population wont be sufficient even when bats require much more food. But I agree that it may strengthen the conclusion of likelihood of lower population.
But then so does option B- If bats are unlikely to eat other insects then non- availability of flying insect concentration might reduce bat population?
Please tell me how am I wrong?
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When mercury-vapor streetlights are used in areas inhabited by insect 20 Jun 2020, 09:28
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GMATNinja or anyone who can help.
Option D- We do not know that the insect population wont be sufficient even when bats require much more food. But I agree that it may strengthen the conclusion of likelihood of lower population.
But then so does option B- If bats are unlikely to eat other insects then non- availability of flying insect concentration might reduce bat population?
Please tell me how am I wrong?
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Hi

Let me try to address your query.

Option (B) states: the bats are unlikely to feed on insects that do not fly.

From the stimulus, we know that:

i) the bats feed almost exclusively around the lights, because the lights attract flying insects.
ii) energy-saving sodium streetlights do not attract insects.

From (i) and (ii), we can infer that there would no longer be a concentration of flying insects around the sodium streetlights. In order for (B) to strengthen the conclusion of lower bat population, it must, in some way, indicate lower feeding possibilities for the bats. Option (B) does not do this completely - it only suggests lower concentration of potential feeding opportunities around the lights. It is very much possible that these insects would still be flying around in the same numbers and thereby be available as food for the bats, just not around the sodium lights.

Therefore, option (B) is not the correct answer. Hope this helps.
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When mercury-vapor streetlights are used in areas inhabited by insect 13 Jan 2021, 05:24
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anchitahuja
Why isn't A the answer? If the bats only hunt after sundown, and exclusively feed on flying insects, they won't be able to catch them under the lights, and hence starve and die?

is it that the GMAT assumes bats are nocturnal, and will be able to hunt the insects regardless of whether the insects gather around streetlights or not?
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GMATNinja, karishma

I too have the same question. Can you address this?
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When mercury-vapor streetlights are used in areas inhabited by insect 21 Jan 2021, 11:22
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Why isn't A the answer? If the bats only hunt after sundown, and exclusively feed on flying insects, they won't be able to catch them under the lights, and hence starve and die?

is it that the GMAT assumes bats are nocturnal, and will be able to hunt the insects regardless of whether the insects gather around streetlights or not?

GMATNinja, karishma

I too have the same question. Can you address this?
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I’m going to borrow heavily from our previous post. (Is it bad to plagiarize myself? I mean, the band Nickelback got in trouble for it...?)

I'll risk it: the fact that bats can no longer catch insects around the lights does not NECESSARILY mean that bats cannot catch insects and will starve. The issue is not that the GMAT assumes bats are nocturnal and will be able to hunt at night. Rather, the issue is that we would have to assume that bats are somehow negatively impacted by the insects changing locations.

The problem with the argument as written is that it does not provide reason to believe that the bats will be negatively impacted. Sure, their food source is no longer gathering around lights, but why is that bad? Merely stating, as (A) does, that bats do not begin to hunt until after sundown does not necessarily tell us the bats are going to be negatively impacted. Instead, indicating that bats must expend more energy and find more food gives reason to think the bats will be negatively impacted and the population will drop.

So we can eliminate (A), and (D) is correct.

I hope that helps!
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When mercury-vapor streetlights are used in areas inhabited by insect 27 Feb 2025, 08:07
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