A, the ignition point of combustion will not change under normal circumstances, so A is wrong;

B, the main role of liquid carbon dioxide in fire fighting has nothing to do with the prevention and control of combustible air pollution, so B is wrong;

C, when liquid carbon dioxide is vaporized, it absorbs a lot of heat, so that the temperature is reduced below the ignition point of combustible materials; Carbon dioxide is denser than air and does not support combustion. It covers the surface of combustible materials to isolate combustible materials from air (or oxygen), thus achieving the purpose of fire fighting, so C is correct.

D liquid carbon dioxide will not decompose into new substances, so d is wrong.

So choose C.