You are facing compiler optimization. In the first run you never used 'C'. The compiler recognizes that, and ignores C, reserving no space for this variable.
This is often the problem if you try such simple programs to see how it works. To switch off optimizing you could add a line at the top of your sketch:
#pragma GCC optimize ("O0")Then the compiler doesn't optimize and the result is nearer to what you expect. But you should never switch off optimizing in a 'productive' sketch.