常用低压电动机的保护元件有哪些？
参考答案：（1）熔断器。（2）热继电器。（3）带有失压脱扣、过流脱扣功能的控制设备。（4）继电保护装置。
She has studied in this school _______ she was seven years old. [ ]
A. since
B. if
C. until
D. after
并联电容器所接的线停电后，必须断开（）。
