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?
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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!
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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

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!
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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.
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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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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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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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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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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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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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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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It seems that that the question asks for a lot of assumptions in the options.
Option A- As insects are not available after sunset and if the bats do hunt after sundown, then number of bats will decrease due to less available food for each bat
Option B- If bats do not eat insects that do not fly, then numbers will decrease
Option D- Bats require much larger quantities of food for each bat, but what if there more than sufficient quantity of insects available for feeding. Then, it will be true. Right?

Can the experts pls help.

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?

Type: Strengthen the conclusion

Conclusion: This change of streetlights is likely to result in a drop in the population of insect-eating bats in Greenville

(A) the bats do not begin to hunt until after sundown - irrelevant to the conclusion drawn.

(B) the bats are unlikely to feed on insects that do not fly - we do not know whether the change of lights attracts other insects.

(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 to the conclusion.

(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 - (D) implies that after such a change is implemented, bats would it increasingly difficult to sustain themselves. As a result, they will be forced to relocate to other areas where food may be available. This change of streetlights therefore leads to a drop in the local population of insect-eating bats. (D) therefore supports the conclusion. Hence, (D) is the right answer choice.

(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 - (E) would somewhat weaken the conclusion, definitely not strengthen it.

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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