当主电路或电动机过载或短路时能自行断开电源的一种安全保护装置叫（）
A、过流继电器
B、过电流保护
C、电压保护
参考答案：B
One of the best teachers in our class________ born in Shanghai.[ ]
A. is
B. are
C. was
D. were
Onvif
