Answer: B

In nearly all arguments based on a plan or proposal, one underlying assumption is that the plan is feasible. When you read the argument, you might assume that it's possible to build a mechanism to do what is suggested and fit it into the overall workings of an automobile.

Alas, choice (B) indicates a very serious weakness: this isn't possible. Choices (A), (C), and (E) slightly weaken the plan. (A) and (C) provide reasons why the plan won't be hugely effective, but they don't cast doubt on the possibility that the plan will have environmentally-friendly results. (E) suggests the plan will be expensive, but without more details we don't know whether it would be prohibitively so. That leaves (B), which most weakens the plan, and is correct.