Do you know     ?

A．what he happened

B．what the matter is with him.

C．what happened to him

D．what did he happened

低压储气罐水封冒气的原因主要有（）。

A.水封槽水位过低

B.大风使罐体摇晃

C.下部塔节被卡

D.上升塔节被卡