Then I'd play in A minor.
Seriously though. Here's the thing. There are instruments unlike the piano, such as the violin, that have variable pitch. - or the human voice....
So, as J_menz pointed out. This question is totally ridiculous.
Well, yes and no. It's ridiculous in that if you have the interval between B and C, someone would likely be able to find C#, but on the other hand, there are other cultures whose music uses quarter steps in addition to half steps. Sure, technically /we/ could use them too, but we don't. I imagine they could have a discussion like this and think, "No, there's no way someone would only have 12 notes in an octave (probably not the word they'd use...). They can bend pitch with various instruments, why not just create the rest of the notes?"
I understand it's a little different, but...
If I actually were in this situation though, I'd just play in C dorian.