单项选择题
Any electric motor can be constructed to be（）.
A.short proof B.ground proof C.explosion proof D.overload proof
单项选择题 Current changing in direction and rising and falling in value is （）.
单项选择题 The output voltage of a three-phase alternator is regulated by the （）.
单项选择题 To repair a small electrical motor that has been submerged in saltwater, you should （）.
