What is the source of this question? There's a good argument for saying that ALL the choices strengthen the argument. (E) strengthens it very clearly: If cars last longer and require fewer repairs because of driving more slowly, then driving more slowly saves money. If fewer accidents occur (which is C, sort of), then driving more slowly saves lives.
I think the ultimate reason why (A) is the official answer is that (A) does not support the conclusion that money is SAVED; it only supports the conclusion that people don't spend MORE. I've seen this pattern before on a few questions.
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