Nitrogen triiodide is a highly explosive chemical that is easy to make

KS15, answer A requires quite a few assumptions in order to serve as an explanation. Do we know that "somewhat restricted" sources are riskier or more difficult to buy from? Do we know that there are other, less risky, options? Couldn't someone steal the iodine? Would these considerations really deter a potential wrongdoer? We can see why it might be mildly challenging to collect the needed ingredient, but that's a far cry from showing us why no one has ever used this substance, and that is the situation we're trying to explain.

Remember that we're not looking for the same kind of answer we'd like in a Weaken q--one that introduces doubt or points us in a particular direction. Rather, we want an answer that clears up our confusion more or less completely. E immediately clears up that confusion. We don't have to assume much to figure that it would be undesirable to work with something that can explode as a result of even slight movement.

I'm not usually a fan of explanations that rely on "this is a better answer" rather than "this is the only right answer at all," since that leaves us open to the idea that the right answer is a matter of opinion. E is so definitive that it keeps me from worrying too much that A could also be the answer, and that's how I would proceed to solve this problem in real-time. However, in review I might notice the issues I mentioned in the previous paragraph and question whether A really does much to explain the discrepancy at all.