From Manhattan:
Step 1: Identify the Question
The word weakens in the question stem indicates that this is a Weaken the Argument question.
Step 2: Deconstruct the Argument
Gen Eng. seeds req. less pest control
$ saved from ↑ > extra seed cost
© Gen Eng. seeds reduce crop cost for farmers
Step 3: Pause and State the Goal
On a Weaken question, the goal is to attack the logic between the premise(s) and the conclusion. In this case, the conclusion is about the overall cost of growing crops, but the premises include components of that overall cost: seeds and pesticide. Look for an answer that identifies another costly consequence of using genetically engineered seeds.
Step 4: Work from Wrong to Right
(A) This answer choice does not address the change in cost that accompanies switching to growing crops from genetically engineered seeds that are available.
(B) This answer choice presents information related to cost; however, this information supports the idea that the genetically engineered option will save farmers money. Seeds that are not genetically engineered require more pesticide, and so the costs associated with using these seeds will rise more than the costs associated with using genetically engineered seed if the cost of pesticides increases. This answer strengthens the argument.
(C) CORRECT. This answer choice identifies a cost that was overlooked in the argument. If genetically engineered seeds require more fertilizer and water than other seeds do, farmers will spend more money on this component of growing crops from genetically engineered seeds, potentially making the conclusion of the argument untrue.
(D) If the cost of genetically engineered seeds were to be reduced in the future, then farmers would have a better chance of cutting costs by growing crops from these seeds. This answer choice strengthens the argument.
(E) This answer choice centers on the irrelevant distinction between crops that have a genetically engineered seed option and those that do not. However, the conclusion is specifically about farmers who use genetically engineered seeds, and is thus only relevant for crops that do have a genetically engineered option available. Therefore, if a farmer grows a crop that requires a great expenditure on pesticides but that does not have a genetically engineered option, this particular farmer is not one of the farmers described in the conclusion, and so this choice has no bearing on the conclusion.