a question

This was asked to me in an interview. I am wondering how many of you know this

What is the type of the addition of an unsigned and a signed integer in C?

>>14
I am still not sure, it was a simple code with a printf in it. I couldn't find anything wrong with it. I guessed that printf is probably are not developped with interrupts or multi threading in mind and it may cause errors. For example if it works in a critical section guarded by mutexes, it could go into a deadlock if it interrupts while printing another thing