单项选择题
An accidental path of low resistance, allowing passage of abnormal amount of current is known as a/an（）.
A.open circuit B.short circuit C.polarized ground D.ground reference point
单项选择题 Which of the following statements represents the main difference between a relay and a contactor?（）
单项选择题 Any electric motor can be constructed to be（）.
单项选择题 The output voltage of a 440 volt, 60 hertz, AC generator is controlled by the （）.
