I think he may have gotten it from the Wrong Answer Fairy. She's a sneaky one...
I had almost posted a reply "well, I can tell you what it's not. It's not 'x' and not 'z' either.". Good thing I didn't, because mistakes like this maximize the potential for learning.
Anyways, like TrickyDicky asked, where did you get that answer? Suppose the answer is not 'x', what do you think it should be?