The argument is designed to establish the conclusion that we should try a new marketing campaign. The evidence for why we should do this is that the product is not selling and that while not guaranteeing success, it's one way one might save the product.
The problem here is that the argument has failed to consider possible reasons why we should not try a new marketing campaign. The drug company manager has named two reasons why we might try a new campaign, but has not thoroughly weighed the pros and the cons.
To weaken this argument we need to find some potential drawback to a new marketing campaign. Answer choice (D) provides a potential negative impact of trying a new campaign, and so represents the correct answer.
What's tougher than seeing how answer choice (D) weakens the argument is seeing how the other answer choices do NOT!
(A) strengthens the argument that something should be done to help out the failing new product.
(B) doesn't tell us anything about the likelihood that trying a new marketing campaign will be successful.
(C) uses a qualifying trigger that is not going to happen. The evidence suggests there is a chance that the marketing campaign will succeed.
(E) is irrelevant to whether the impacts of a new marketing campaign for the new product would be positive or negative to the company.
Let me know if you still have questions on this one!
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