Source: Official Guide for GMAT Review 2016 Critical Reasoning; #65

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# Which of the following, if true, most strengthens

Which of the following, if true, most strengthens the argument?

### 1 Explanation

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Felix Manan

Hi, was wondering if I can get assistance on this question.

I know that premises are facts and those can't be challenge, whereas conclusions can. I was wondering why A and similarly C cannot be the answers.

The conclusion is "so it is likely that DS from garlic repelled the mosquitoes" (C&E). I know it's the conclusion because of the indicator "so" and it is supported by the previous statement "DS is known to repel insects, mosquitoes included".

In A I am given that onions has DS. From the premise I know that DS repels mosquitoes, so why can't I say Onions repels mosquitoes, strengthening the conclusion because while it's something different, it is a similar situation - both contain DS? The OA even mentions that we don't know whether onions repel mosquitoes. The similar logic can be applied to C. The only justification that I can provide is that the argument's flaw plays to circular reasoning. Premise itself used to justify the conclusion is the conclusion. Is this correct? Thanks!

Apr 12, 2018 • Comment

Sam Kinsman

Hi Felix,

A) tells us that "DS is also found in onions but at concentrations lower than in garlic." This would make us think that onions might also repel mosquitoes. But this fact by itself doesn't strengthen the conclusion. If we know that onions also have DS, this doesn't make it clearer that the DS from the garlic repelled the mosquitoes. Keep in mind that onions and garlic are two different things, and onions are not mentioned anywhere in the question. So if we know that onions also have DS, that doesn't really help us strengthen the conclusion.

"By the end of the summer, most of the garlic plants in the ﬂooded ﬁelds had been killed by waterborne fungi."

Again, this doesn't really help us strengthen the conclusion. If the garlic plants were killed, perhaps that means there was no more DS in the water - but we cannot be sure. Even if there was no more DS in the water, that still doesn't give us any additional indication that it was the DS that repelled the mosquitoes. So C does not strengthen the conclusion.

The argument does not really have circular reasoning. Also, keep in mind that we want to strengthen it, and not weaken it!

Let's look at D:

"Many insect species not repelled by diallyl sulﬁde were found in the ﬂooded garlic ﬁelds throughout the summer."

This tells us that those species NOT repelled by DS were found in the flooded fields. Aha! That means that there wasn't some other, non-DS related reason why the insects weren't there. So this further suggests that the reason why mosquitoes weren't there was the DS. So this strengthens the conclusion.