The argument says that the higher the demand for something, the easier it is to sell. It proceeds to suggest that there is more demand for car parts that are less than ten years old than those that are ten years or older, so the younger cars are generally easier to sell. This is a strengthen question with a necessary premise ("assumption on which the argument depends"), so we know that the negation of the correct answer will bring about the falsity of the conclusion.
Answer choice E is the correct answer because it's negation- "the salability of cars that are ten years old or older is NOT largely a function of the level of the demand for their parts" -causes the argument and its conclusion to fall apart. It suggests that there are other, significantly more important factors to be considered in determining how easy it is to sell the cars, and no other such factors are considered in the argument.
Answer choice A is incorrect because it does not as significantly change the argument when negated. Negated, it reads "the salability of something IS influenced by at least one factor other than the demand for it." Although this is on the right track, it does not specify that this other factor is at all significant, just that it exists. Therefore, it is not completely incompatible with the conclusion.
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