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?

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

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.
_________________

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.
_________________

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.
_________________

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.
_________________

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.
_________________

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.
_________________