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 The output voltage of a 440 volt, 60 hertz, AC generator is controlled by the （）.
单项选择题 A ground can be defined as an electrical connection between the wiring of a motor and its（）.
单项选择题 Brushless generators are designed to operate without the use of（）.
