Answer
A=compound
B=not as certain, but likely a compound.
C=compound
Work Step by Step
We know that A gives off the gas C when heated w/o oxygen, and that gas C is actually C$O_{2}$. This means that A and C are both compounds, as A has to have at least carbon and oxygen in it to begin with. Substance B isn't as certain, but very few elements naturally occur as white solids, so it is very likely substance B is a compound.
