As Millie_____ TV, Andy _____ the door. [ ]

A. was watching: was opening

B. watched; opened

C. was watching; opened

D. watched: was opening

一台X线机带有两只X线管时，完成X线管切换的部件是

A．高压交换闸

B．继电器

C．限时器

D．高压电缆

E．电源开关