Have you ever met with this case in your life 　　　　 you can't understand others

even if they have said something several times？

A．where

B．what

C．that

D．which

封闭式负荷开关操作机构有（），以确保操作安全，避免发生触电的危险。

A.延时装置

B.综合闭锁装置

C.电气联锁装置

D.机械联锁装置