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I am having a very hard time with this one, and the forum search did not find anything on this.

People who do X are much more likely to suffer from A than people who don't do X. Doing X involves stresses. Thus, it is likely that these stresses cause A.

We need to strengthen the argument, and here is answer:

people who already suffer from A are no more likely than people who do not to accept assignments from their employers that require doing X.

Can anyone explain what is the point here, and how exactly it strengthens the argument?

This is a causal conclusion: business travel causes insomnia. Causal conclusions are really difficult to prove because there could be something else causing the insomnia, or it could be that the cause and effect are reversed. In order to strengthen a causal conclusion, you can (1) eliminate alternate causes, (2) point to another time where the same cause and effect happened, or (3) point to another time where you didn't have the cause and thus didn't have the effect.

In this case, it could be that people who already suffer from insomnia decide to accept projects that require a lot of travel, because they already have difficulty sleeping so taking red-eyes, etc., isn't a problem. (So it's not that the travel causes insomnia; it's that people with insomnia are predisposed to this kind of travel.)

(C) strengthens the argument by ruling out the possibility that the cause and effect could be reversed.

Make sense?

I got it.

Another example of the cause-effect, or an elimination of a possibility that could undermine it - that's what I should have pre-phrased before looking for an answer choice, otherwise it's very tough to figure it out...

Thanks!