单项选择题
You are attempting to parallel two AC generators and the synchroscope pointer is revolving in the slow directionThis indicates that the frequency of the incoming machine is （）.
A.higher than the bus frequency B.lower than the bus freouencv C.the same as the bus frequency but out of phase with it D.the same as the bus frequency, and the circuit breaker may be closed at any pointer position
单项选择题 The need for insulation cleaning may be determined by （）
单项选择题 Voltage will always lead current in a/an （）
单项选择题 The number of cycles per second occurring in AC voltage is known as the （）
