单项选择题
Prior to closing the breaker when paralleling two AC generators, the recommended practice is to have the frequency of the incoming machine （）
A.slightly less than the line frequency B.the same as the line frequency C.slightly greater than the line frequency D.All of the above
单项选择题 Which of the following should be the FIRST step in removing a generator from parallel operation?（）
单项选择题 Electrical power is expressed in （）
单项选择题 Which of the following materials is a good insulator?（）
