30. When he was only thirty years old, he had already experienced______most people only experience when they are much older.

A．what

B．which

C．who

D．When

调速系统设置附加保安油装置的目的是，机组转速超过危急保安器动作转速而达到附加保安动作转速时，使危急保安滑阀动作停机。（）