—Look! The telephone is broken. Someone damaged it       purpose.

—That may be right. But perhaps it was broken       accident.

A．on; by

B．by; by

C．on; on

D．by; on

三相异步电动机的反接制动，方式简单、可靠，但振动冲击力大，一般适用于（）4kW的电动机且启动不太频繁的场合。

A.大于

B.等于

C.小于

D.都可以