Based on oxidation number consideration, explain why carbon monoxide is a flammable but carbon dioxide is not

Carbon in carbon monoxide has oxidation number + 2 and can be oxidized to oxidation number + 4 (carbon (IV) oxide) in reaction with oxygen:

2CO + O₂ → 2CO₂.

Carbon has maximal oxidation number in carbon (IV) oxide and cannot be more oxidized (increase of oxidation number), carbon with oxydation number + 4 cannot lose electrons to molecule of oxygen.