When mercury-vapor streetlights are used in areas inhabited by insect

Question

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

ID: 500271

Kurtosis · Accepted Answer

When Mercury vapor street lights were used - Bats fed insect around the light
Mercury vapor streetlights are now replaced by sodium streetlights - Does not attract insects - Bats population will drop

We have to select an answer choice that will provide the reason for the drop in bat population

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

(B) the bats are unlikely to feed on insects that do not fly - Incorrect - Rather than providing the reason for the decline in bat population this option tries to attack the fact stated in the argument.

(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 - Incorrect - Irrelevant

(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 - Suppose there were 100 bats and 100 insects, earlier 1 bat required 1 insect. Because the light does not attract insects bats spend more energy now. So 1 bat requires 2 insects. 100 insects is sufficient for only 50 bats. Other 50 will starve to death, resulting in population decline.

(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 - Incorrect - Again attacks the fact stated in the argument

Answer: D

chetan2u · Answer

Respoding to a PM,

lets see the PARA-
 Bats feed EXCLUSIVELY on the insects around the street light. These street lights are now being converted to some other form which does not attract light. 
The conclusion is "This change is likely to result in a drop in the population of insect-eating bats" and we have to complete the statement citing some reason for it..

What are you required to do-   Give reasons for the conclusion..

What does it ask you to do?
 Your answer has to be -
1) firstly related to some info existing in the PARA.
2) secondly it has to be strengthening the conclusion.

lets see the choices for these two points-

(A) the bats do not begin to hunt until after sundown 
 Does it give reason to strengthen our conclusion. NO..

(B) the bats are unlikely to feed on insects that do not fly 
 Again nothing .. the point would be same for any kind of street lights..

(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 
 It does not follow any of our 2 points
Out of context

(D) in the absence of local concentrations of the flying insects on which bats teed, the bats expend much more energy on hunting for food, requiring much larger quantities of insects to sustain each bat 
 Now this is related to change over of lights..
Also that word in capitals EXCLUSIVELY tells us that they were having easy meals. But this changeover has resulted in two things-
a) Lesser avail of insects in close proximity 
b) spending extr aenergy for food and in turn requiring MORE food. likely that the strength will go down
PERFECT as an answer

(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
 again Not related to the conclusion

ChiranjeevSingh · Answer

Understanding the Passage

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

This statement talks about areas inhabited by insect-eating bats. When mercury-vapor streetlights are used in such areas, the bats feed almost exclusively around the lights.

because the lights attract flying insects.

This part gives an explanation for why bats feed almost exclusively around the light. Because the lights attract flying insects. (This perhaps makes it easier for bats to prey on insects)

In Greenville, the mercury-vapor streetlights are about to be replaced with energy-saving sodium streetlights, which do not attract insects.

In Greenville, the above type of streetlights are about to be replaced with streetlights that do not attract insects. (In such a case, the insect-eating bats will have to look for their food elsewhere.)

This change is likely to result in a drop in the population of insect-eating bats in Greenville, since __________.

The change of streetlights from those that attract insects to those that don’t attract insects is likely to result in a drop of insect-eating bat population.

The statement ends with “since”. Thus, we’re looking for an option that gives us a reason for why the change will lead to a drop in the insect-eating bat population.

Some Thoughts

Would the following option be correct?

The government of Greenville is taking several measures to reduce the population of insect-eating bats.

The answer is that this option is not correct.

Why?

Because while this option gives us a reason to expect a decline in the population of insect-eating bats, it doesn’t give us a reason to believe that the change in streetlights will lead to a decline in the population of the bats.

In other words, we’re looking for a reason for how X will lead to Y. Knowing the presence of other things that can lead to Y will not impact our argument.

For example, if I say, “If Raj studies daily, Raj will increase his chances to succeed on the GMAT”, this statement cannot be impacted by saying “Raj will take the help of a good GMAT tutor”. While taking the help of a good GMAT tutor is expected to increase Raj’s GMAT score, this does not impact whether studying daily will increase his GMAT score.

The Evaluation

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

Incorrect.   This option has NO IMPACT on the last statement (the change in streetlights -> drop in the bat population).

This option says that bats do not hunt before sundown.

Let me create a more direct variation of this option:

A1: The bats hunt only when streetlights are on.

Does A1 have an impact on the last statement?

No. Whether the bats only when streetlights are on or whether they hunt even when streetlights are off has no impact on the last statement (given that the passage says that the bats feed almost exclusively around the lights). The change in streetlights is still expected to decrease the population of bats.

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

Incorrect.   This option has NO IMPACT on the last statement (the change in streetlights -> drop in the bat population).

This option indicates that the bats don’t eat non-flying insects, i.e., they feed exclusively on flying insects.

This option would have explained the last statement (and thus would have been the answer) if the passage had indicated that the change in streetlights is expected to reduce the population of flying insects. However, the change in streetlights is not expected to reduce the population of the flying insects.

Thus, this option doesn’t explain why the change in streetlights will reduce the bat population.

(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

Incorrect.   This option has NO IMPACT on the last statement (the change in streetlights -> drop in the bat population).

Whether the replacement takes a small amount of time or a long amount of time and whether there is a disruption in lighting at certain locations or not HAVE ZERO RELEVANCE to the last statement.

(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.    This option explains how the change in streetlights will lead to a drop in the bat population.

The option indicates that in the absence of mercury-vapor streetlights, the bats will require much larger quantities of insects to survive.

If so, we have a reason to believe that some bats will not find enough food, given that the insect population is not expected to increase.

In such a case, we have a reason to believe that the bat population will decline as a result of the change in streetlights.

(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

Incorrect.   This option has NO IMPACT on the last statement (the change in streetlights -> drop in the bat population).

This option would have argued against the drop in the bat population if we were removing mercury-vapor streetlights and not putting any streetlights in place of them.

If bats gain no advantage from the visibility of the insects, we get an indication that removing the streetlights will not hamper their success in hunting insects. Thus, we have a reason not to expect a drop in the bat population.

However, this reasoning does not apply to our given context since we’re just changing the type of the streetlights; we’re not removing the streetlights completely. The visibility is not changing in the given situation. Thus, the option is irrelevant to the given situation.

Abhishek009 · Answer

Mercury-vapor streetlights----> Attracts insects -----> Bats prey on them around the lights (at a single location)

No Mercury-vapor streetlights -----> No insects around the lights -------> Drop in population of bats

So, how does Bats survive ? Does they stop eating insects ? Probably not , they must be hunting the insects anyway for survival.

(A) the bats do not begin to hunt until after sundown - Talks about sundown , but doesn't account for the Bats food and reduced population.

(B) the bats are unlikely to feed on insects that do not fly - Out of scope.

(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 - Talks about replacement of llights and not about the reduction in population of bats.

(D) in the absence of local concentrations of the flying insects on which bats teed,  the bats expend much more energy on hunting for food , requiring  much larger quantities of insects to sustain each bat  -   This can be a cuse for reduction of Bats population.

(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 - Out of scope.

Hence answer is (D)

anchitahuja · Answer

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?

zoezhuyan · Answer

Hi  mikemcgarry ,  GMATNinjaTwo ,  GMATNinja ,  MagooshExpert  Carolyn, 
 sayantanc2 
 VeritasPrepKarishma 
 
First, i view this question as an assumption questions. so i need get a unstated premise that author believe it can lead to the conclusion.
i don't understand how D leads to the conclusion, i think i must miss something.
D says, in the area where lack the concentration of flying insects, bats need to spend more energy to hunt, 
here is the question, how bats spend  more energy  will lead to drop population.
 more energy  means it is harder for bats to hunt, but they highly possible to stay in Greeville, then the population does not necessary drop,

Please help point out.
Have a nice day
>_~

MagooshExpert · Answer

Hi zoezhuyan ! Happy to help

The idea here is that with the old lights, there are lots of insects in one place (near the lights). That makes it really easy for bats to find food -- all they have to do is go to the lights, and they will find lots of insects to eat. With the new lights, the insects will be spread out all over the place. That means that it will be harder for the bats to find insects to eat, since now they have to fly around and look much harder for the insects. That means the population of bats will drop, since it's harder for them to find food. Does that make sense? If not, let me know

-Carolyn

GMATNinja · Answer

I know that I'm late to the party, but let's see if this helps at all:

Choice (D) specifically tells us that when bats expend more energy on hunting for food,  much larger quantities of insects  are required to sustain each bat. In other words, the bats will need more energy, so they will need to eat more insects. But the number of insects is not expected to change. The current insect level is enough to feed the current bat population. If the size of both populations remains the same but the number of insects required per bat increases, then some of the bats will not have enough insects to eat. Thus, some of the bats will not survive, and the bat population will likely drop.

zoezhuyan · Answer

Hi  GMATNinja , I can understand what you explanation,
i have one more question, how can i get the information  The current insect level is enough to feed the current bat population.

Would you please clarify further?

Have a nice day
>_~

adkikani · Answer

GMATNinja   VeritasPrepKarishma

We are just given that special types of bats feed ALMOST EXCLUSIVELY on insects present in areas around the light. How did we link FEEDING on insects to one of such extreme assumption of NOT SURVIVING without feeding on insects as in OA?

GMATNinja · Answer

Thanks  dave13  for the explanation!

Also, keep in mind that the conclusion is that "this change is   likely   to result in a drop in the population." We do not need something that PROVES that the population will drop. As explained perfectly by  dave13 , choice (D) tells us why the change will LIKELY result in a drop in the bat population.

zoezhuyan , sorry for the delay...

Regarding the portion you highlighted, I was simply referring to the pre-change bat population level. Regardless of what that level is, we can say that there are currently enough insects to sustain the  current  population level. Assuming that nothing changes, the status quo will remain the same. But if bats suddenly need more energy (as explained by  dave13 ), then there might not be enough insects to meet the increased demand for food.

I hope that helps!

CAMANISHPARMAR · Answer

GMATNinja  - I feel the logic is related to non availability of flying insects in concentrations rather than pre & post change in insect population? The passage is silent on whether the insect population is sufficient or about their reproduction rates? Please clarify.

BrightOutlookJenn · Answer

Hi  CAMANISHPARMAR

You asked me to weigh in on this one so I will give you my thoughts. There are already some great explanations above.

Basically, we are looking for something that describes why replacing these lights is going to be a BIG PROBLEM for the bats. 
It needs to be a life-threatening problem, because the argument says the change will likely result in a  population decline . Population decline means existing bats die off and/or the next generations of bats are smaller.

What could cause such a thing? 
Hmm, 2 big categories come to mind:  a shortage of any of the essentials  a bat needs to survive (food, nesting grounds, etc), or the  presence of a killer  such as a disease or a predator. (A natural disaster would be another option, though I don't foresee that coming into play here.)

So I could go for any answer that gives me a shortage of essentials or the presence of a killer.

For best results, we have this all in mind BEFORE we go shopping through the answer choices for an answer that meets our needs.

No answer choices give us a killer. And only one of our answer choices could lead to a shortage of essentials. D describes that bats are going to be working way harder than before to find enough food to eat, and as a result, they're going to suddenly need more food. If they can't find that additional food, they will die ... and/or not be able to feed the same number of baby bats as before, thus reducing the next generation.

It is not 100% sure that they won't be able to find additional food, but as  GMATNinja  and  dave13  have mentioned, 1) we don't have to be 100% sure (the question says "likely"), and 2) there is a finite existing insect population. It's reasonable to envision that there will not be enough insects to feed the bats' increased appetite.

Let me know if this helps.

jayarora · Answer

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

What is wrong with option (A) here? If they don't begin to hunt till the sun is set and at night they are not going to get a concentration of flying insects, can't we say they will starve and hence the population is likely to reduce?

GMATNinja · Answer

You are correct in stating that the bats are not going to get a concentration of flying insects when the new lights are installed, but we cannot infer from this information that the bats will starve.

Even if the bats only hunt at night and flying insects do not congregate around the lights, the information in (A) does not say that the  actual number  of flying insects will decrease. So the bats won't have their convenient flying insect buffet near the lights, but it is possible that they could find just as many insects to eat in other locations. For this reason, (A) is not the correct answer... and the bats will probably survive.

I hope that helps!

rnn · Answer

hi all... what is wrong with option (B) the bats are unlikely to feed on insects that do not fly ?

if the bats don't feed on insects that do not fly then changing the light source would mean that no flying insects would go near the light

GMATNinja · Answer

It is true that the new lights will not attract flying insects like the old lights did. However, we cannot infer from this that the actual number of flying insects in Greenville will decrease -- we just know that they will not be congregated so closely around the lights. Our task is to find additional evidence in the answer choices to support the claim that changing the lights "is likely to result in a drop in the population of insect-eating bats in Greenville."

Answer choice (B) tells us that "the bats are unlikely to feed on insects that do not fly." Because we cannot say that the actual number of flying insects will decrease when the lights are changed, this answer choice does not provide good evidence that the bat population will drop. The bats will continue to eat flying insects and will survive just as they did before.

I hope that helps!

Chelsea212 · Answer

Hi everyone! I'm very late to this party, but seeing as it seems to be a fairly controversial (can't think of a better word, ha.) question, I thought it might be helpful to contribute another perspective. Also, I got this question incorrect so it's a reinforcing habit to post my learnings on here. I will put a focus on explaining my reasoning as opposed to knocking out a choice with the phrase 'out of scope' as I personally find that to be the most unhelpful comment ever.

Generally on CR, I prefer to read the question first and jump into the stem. I read fairly quickly, so I'm not concerned about the fact that I may end up reading the stem twice by starting in their order. In my experience, knowing the question shifts the perspective from which I approach reading the stem.

1. Read and compartmentalize  - I read the question, read the stem, and mentally make note of what role each sentence plays: premise, fact, conclusion. If there is a conclusion - which there usually is - you absolutely must recognize it and keep it in mind as you weed through the answer choices. Chances are, if and when you answer a question incorrectly, it is because you did not understand the conclusion.

2. Prethink when possible - always!  - Sometimes you really can't rethink, and that's okay. But you will probably find that through dedicated practice, you'll usually be able to 'rethink' more options more often. For this specific question, my 'prethink' points include - predators of bats, some reason they die.

3. Find 4 wrong answers  - I interpret this question to be almost a parallel reasoning type of question. The word immediately before the blank is 'since' and so I'm focusing on finding a premise to support the conclusion, which is the first clause of the last sentence.

(A) the bats do not begin to hunt until after sundown   - If this were the case, the changing of streetlights would have no impact on the population of insect-eating bats, and therefore we would not have sufficient reason to infer that their population would drop.

(B) the bats are unlikely to feed on insects that do not fly   This is the one that got me. While bats may indeed be unlikely to feed on insects that do not fly, all of these flying insects do not have to be flying exclusively around the light. The flying insects just happen to be in greater concentration around the light and more readily visible because of it.
In other words, bats eat flying insects and these flying insects exists in a much bigger space than just around streetlights.

(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   - I don't think I saw anyone having difficulty ruling this one out but regardless, this statement actually works against our logic. If the change is quick, then there would be no reason to say, have the bats go long without an easy location for food.

(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   Bingo!  - This nails it right on the head. If any animal has to expend  much  more energy on getting food and hence requires  much  larger quantities of it, then one can absolutely infer that some of those guys would drop dead.

(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  - This statement destroys the entire stem. If they never benefitted from the original streetlights, then why would they all of a sudden drop dead ini large number post switching them out?

A valuable piece of advice I was once given. As you practice, you'll gain a firm grasp on the concept that there is only  one  bona fide correct answer and no such thing as  kinda right  answers. If you understand a question and what it is asking well, the answer will become very obvious to you. Just as there is a black and white right versus wrong answer on the Quant section, there is also the exact same outcome in CR.

kornn · Answer

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

When mercury-vapor streetlights are used in areas inhabited by insect

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When mercury-vapor streetlights are used in areas inhabited by insect

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