Why should you wear a self-contained breathing apparatus before entering the engine room after the fixed CO₂ system has been discharged to combat a major fire?（）

A.Because carbon dioxide dilutes the oxygen concentration in the atmosphere and may cause asphyxiation
B.Because carbon dioxide breaks down in a fire into carbon monoxide which may cause blood poisoning in the person breathing this gas
C.Because carbon dioxide is a colorless and odorless gas that becomes highly toxic in the presence of high temperatures and will quickly incapacitate a person exposed to this gas
D.This action is unnecessary as carbon dioxide is not poisonous nor toxic but is a relatively harmless gas