Ok, I have to admit I didn't know there was a 0th law of robotics and I've read a LOT of Asimov (but it's been a while). I just figured they were counting like geeks do (always start with 0! )

I didn't know there was, either. I'm guessing it came from either a later Asimov work, or from someone other than Asimov. I don't know if I got the answer right, but my guess was based on the fact that the other questions contained 1, 2, and 3, so therefore it couldn't be one of those.