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