One summer, floods covered low-lying garlic fields situated in a region

Mosquito Repellent

Step 1: Identify the Question

The phrasing strengthens the argument indicates that this is a Strengthen the Argument question.

Step 2: Deconstruct the Argument

normally: floods attract mosquitoes

BUT: no mosquitoes in flooded garlic fields

© DS (in garlic) repelled mosquitoes

Step 3: Pause and State the Goal

The goal is to eliminate any answer choice that does not strengthen the conclusion. The right answer will make it more likely that the diallyl sulfide repelled the mosquitoes. The answer might do this by showing that the mosquitoes weren’t actually repelled by some other, unknown factor.

Step 4: Work from Wrong to Right

(A) If the argument stated that mosquitoes also avoided flooded onion fields, this answer would strengthen the argument, since it would suggest that the mosquitoes’ behavior was due to something that onions and garlic have in common. However, the argument doesn’t provide any information about onions related to mosquitoes, so a comparison between onions and garlic doesn’t affect the conclusion.

(B) This answer choice provides another reason for the absence of mosquitoes: there were significantly fewer of them in the area. If this other factor was actually to blame, it’s less reasonable to claim that the diallyl sulfide repelled the mosquitoes. So, this answer choice weakens, not strengthens, the conclusion.

(C) According to this answer choice, most of the garlic plants had died by the end of the summer. This does not have a strong effect on the conclusion, since it isn’t clear how long the garlic plants survived, or what happens to the diallyl sulfide in the plants when they die. In addition, this tends to weaken the argument more than it strengthens it. If the garlic plants weren’t even in the field for a portion of the summer, it’s less likely that their presence was what repelled the mosquitoes.

(D) CORRECT. The argument states that mosquitoes, which are repelled by diallyl sulfide, were not found in the fields. The answer choice adds the information that other insects, which are not repelled by the substance, were found in the fields. So, there is a direct relationship between being repelled by diallyl sulfide, and being repelled from the fields described in the argument. This provides evidence that diallyl sulfide was responsible for the effect as opposed to some other factor that might repel all types of insects.

(E) This answer choice attempts to strengthen the claim that mosquitoes are repelled by garlic. However, the argument states as fact that garlic repels mosquitoes. Since this is already known to be true, adding additional evidence for it does not strengthen the argument. The conclusion of the argument is that the daillyl sulfide in the garlic is what repels the mosquitoes; this answer does not clarify what role diallyl sulfide plays.