A and X are carbon dioxide, and the chemical formula is CO2, not calcium carbonate, so A is wrong;

B, C and CO are flammable and can be used as fuel. Correct.

C, in order to realize the transformation of CO→X, chemical combination reaction and redox reaction can occur, so this error;

D, in the classification of substances, CO and H2CO3 belong to inorganic substances, which is wrong.

So choose B.