（）是利用触点的断开或闭合，自动截断或接通励磁电流，从而改变磁场强度，达到调节发电机电压的目的。

After an explosion，repair of emergency machinery and services should be accomplished（）.

A.after control of fire，flooding，and structural repairs

B.immediately，before the emergency is under control

C.after control of fire，but before control of flooding

D.after stability is restored