A student solved the problem by plugging in different values for x and y (how I also solved it initially), which Greg approved of. However, is it right to say that D is the right answer just because no information was given about y? Literally, “cannot be determined.”

No, because the relationship could indeed be such that the answer is *not* D even if you are not given any information about *y*.

Example: what if Quantity B was |x + y| + 1 instead?

Alright