Capuchin monkeys in Venezuela often rub a certain type of millepede into their fur. Secretions of these millipedes have been shown to contain two chemicals that are potent mosquito repellents, and mosquitoes carry parasites that debilitate the capuchins. The rubbing behavior is rare except during the rainy season, when mosquito populations are at their peak. Therefore monkeys probably rub millipedes into their fur only because doing so helps protect them against mosquitoes.
Which of the following would be most useful to determine in order to evaluate the argument.

A) Whether the two chemicals provide any protection for millipedes against their own repellents.
B) Whether the type of millipede used by the capuchin monkeys in Venezuela is found in other parts of the world.
C) Whether animals other than capuchins rub insects of any kind into their fur
D) Whether the only time millipedes are readily available to capuchins is during rainy season
E) Whether secretions of any other insects accessible to capuchins contain chemicals that repel the mosquitoes"

Source GMAT Prep

Question: Can you please reason out D/E.


The correct answer is : E

I tried negation and was able to derive at an answer but was still now sure.

MGMAT tutors please help.

Thanks
Saurabh Malpani

perhaps the most potent word in the original problem is 'only', which implies that the mosquito-repellent properties are the only redeeming quality of the millipedes described in the passage. in order to strengthen the argument, we need to eliminate alternative possibilities (other reasons why the millipedes might be useful to the capuchins).

it seems you're clear on why a-b-c are irrelevant.

choice e:
consider the consequences of the 2 different answers to this question:
if there are different insects with repellent properties --> this severely undermines the argument, because there must be some other reason why the capuchins are choosing the millipedes exclusively
if there are not different insects with repellent properties --> this doesn't really strengthen the argument, other than indirectly (i.e., by ruling out the possibility of undermining as shown above)

choice d:
consider the consequences of the 2 different answers to this question:
if mosquitoes are available year-round --> this mildly strengthens the argument (by showing that the capuchins really do select the millipedes only during mosquito season, rather than just 'taking what they can get'). but only mildly, because it doesn't really do anything to rule out alternative explanations for the use of the millipedes. (specifically, there could be lots of other uses for the millipedes at that time of year, too).
if mosquitoes are not available year-round --> very mild weakening effect, because this evidence supports the idea that the monkeys just 'take what they can get'

choice e is better because it specifically deals with eliminating alternative possibilities, which is the only really good way to strengthen/weaken a statement saying that 'x is the ONLY reason for y'.

Ron:

I am still confused as to why (A) is irrelevant. The conclusion of the argument is monkeys probably rub millipedes into their fur only
because doing so helps protect them against mosquitoes.

A) Whether the two chemicals provide any protection for millipedes against their own repellents.

If they do not provide any protection, then the argument is weakened. (probably they are doing so for some other reason)
If they provide some protection, then the argument is strengthened.

thanks for your help.

that's correct: the conclusion of the argument is that the monkeys rub the millipedes into their fur because the monkeys are thereby protected against mosquitoes.

choice a deals with whether the millipedes themselves are protected against chemicals, a consideration that is irrelevant to the situation of the monkeys.

Ron, I am confused...I understand ur explaination for choice e but what happened to choice d? In choice d they don't talk about mosquitoes but about millipedes...also the correct answer is D (as per GMATPrep) but u seem to lean towards E...i am confused...please clarify...thanks!

Yes I am sorry the correct answer is indeed D. I must have typed it wrong!

Sorry for the confusion!
Saurabh Malpani

well, ok. :)

note the explanations for choice d, which still stand: apparently, the difference between the strengthening and weakening events mentioned is considered more significant than the difference between possibilities for choice e.

same explanations, but just ignore the word 'mild'.