A. Because the information in the file is more valuable, if foam extinguisher or dry powder fire extinguisher is used to extinguish the fire, the purpose can be achieved, but it is easy to damage the information. It is best to put out the fire with liquid carbon dioxide, because carbon dioxide will spread everywhere after the fire is put out without destroying the data and making mistakes;

B, electrical appliances should use dry powder fire extinguishers, not foam extinguisher, wrong;

C, carbon dioxide fire extinguishers are suitable for fire fighting in cultural relics museums and archives, correct;

D, when the oil depot is on fire, you can't just use the water gun to put out the fire, which is wrong;

So choose C.