Leigh, how can you tell that B is the correct answer?
I tell by noting that this is a memorized factoid that was included
in the course. Why does anyone doubt that? I certainly am not
defending the question; I think it is atrocious, but I am a test
sophisticate (I got all the answers right on the only physics GRE I
ever wrote) and I can spot the desired answer on a question like
this with no trouble at all. The object is to get the desired
answer, is it not?