Many stn/wkn questions, including this one, can be viewed as "explain the phenomenon" or "causal" arguments.

Here, the phenomenon (or effect) is that some people (ie, syneshesiacs) report that they are able to smell colors, etc.

The author's explanation (or cause) is that the five senses are not distinct--that they overlap.

In such arguments one assumption the author always makes is that there are no other explanations.

Because this is a weaken question, the correct answer will tend to refute the assumption, thereby making the conclusion less likely to be true.

How do we attack the assumption that there are no other explanations? We find a choice that suggests there is another explanation.

Choice A tells us that synesthesiacs demonstrate a general impairment in their ability to use and understand words. This points to an alternative explanation: When they report that they are "smelling blue" what they might really mean is that they are "seeing blue".

Thus, choice A weakens the argument, and we can select it without even looking at the other choices. (We don't care about wrong answers and why they are wrong, and we certainly don't want them to eat up our precious time!)

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...Anyways, a very common wrong answer type in weaken questions is a choice that does the opposite--a choice that strengthens.

Looking at the other choices:

Choice B--the possibility of other senses still doesn't give us any information that helps us in evaluating whether senses overlap. Thus, choice B is irrelevant.

Choice C--if there is a definite pattern to the particular ways in which sensory experiences overlap, then the likelihood that senses overlap is definitely strengthened. Thus, choice C strengthens the argument.

Choice D--what people believe to be true is irrelevant in evaluating or strengthening/weakening an argument (and that's a good takeaway). Thus, choice D is irrelevant. Alternatively, if the legends are factually correct, then, if anything, the likelihood of senses overlapping is rendered more likely to be true; on this view, choice D strengthens the argument.

Choice E--if synesthesiacs can temporarily remove synesthetic experiences pharmacalogically, then it suggests that there is a biological basis for synesthesia, and makes more likely the idea that senses can indeed overlap. Thus, choice E strengthens the argument.

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Almost everyone is aware of "explain the phenomenon" or "causal" arguments. But the quick points in CR stn/wkn questions go to those who rapidly recognize when a particular argument fits the form and then go on to characterize the phenomenon and explanation, generating an accurate predition, and, finally, scanning for the matching answer choice. Weaken questions are the most common question type in CR, and explain the phenomenon is the most common pattern in strengthen/weaken questions. You should review these kinds of questions thoroughly until you are comfortable in recognizing the pattern.