单项选择题
Before doing maintenance on electrical equipment, you should （）.
A.turn off power first B.turn on power first C.with a test pen in hand D.tell C/E
单项选择题 While the parallel operation, （） must be used.
单项选择题 When the bearing becomes dry from lack of oil, the motor shaft may heat to （）an extent （） it welds itself to the bearing.
单项选择题 An indicator on the main switchboard should show （） the emergency battery is in service or not.
