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Q. 16. With (A), I think you might be overanalyzing. Sure, the LSAT could have added the phrase "to win long-distance races" at the end for clarification. However, in context of the rest of the answer, that seems implied.
The more important issue is how (A) parallels the stimulus and (D) fails to do so. In the stimulus and (A), lacking two characteristics tends to cause something to not happen. In the stimulus, lacking convenience and variety leads to little success. In the (A), lack of speed and endurance lead to few races won. This also happens in (D), in which lack of firmness and flavor leads to unsuitability for baking.
Then, in the stimulus and (A), the author shows how having just one of the two characteristics is sufficient, concluding that you don't need both. (D), however, changes the topic from baking apples to eating apples. That's not parallel, and is therefore incorrect.
Q. 17. I have to agree with you, (C) is a pretty poor weakener. However, unlike (D), (C) does address the heart of the argument. The argument is basically saying "it's either an accountant or an actuary; it's probably not an accountant so it's probably an actuary."
Initially, the author claims that either employee is likely to embezzle. Then, he only offers one reason against it being an accountant. So, (C) tips the scale in the other direction, giving one reason why an accountant would be more likely (after all, (C) suggests there are four times as many accountant suspects).
(D) merely suggests that embezzlement was likely, but offers not support or counterevidence to the author's claim about the culprit being an actuary instead of an accountant. Therefore, (D) does nothing to the argument, and cannot -- in any way -- be considered a weakener.