This smacks of a trick question to me. If you've been using the language for long enough, you've probably run into the problem and know the answer; but how long is long enough?
It's also a question where your background might work against you. I know in C++ the definition in B would hide the definition in A, but I have no idea if it works the same in C#. In real life, I'd know it was a questionable area and try to look it up. In an interview I might try to guess, having been put on the spot. And I'd probably get it wrong.
