Hi there,
Perhaps someone will (and should) give a more technical answer, but I look at it this way.
For 1), cross-multiply to get: pxq=qyp. You can simplify by dividing each side by "p" and "q," which produces x=y. Thus, 1) is sufficient, because we can mathematically answer the question.
2) xy=p doesn't tell us much. We can determine that the product of "x" and "y" equals "p," so "x" and "y" are probably smaller values than "p," but that's useless in answering the question. So, 2) is insufficient.
The answer is A because 1) alone is sufficient to answer the question, but 2) alone is not sufficient.
