单项选择题
A switchboard, for a AC electrical distribution system, will be provided with which of the following components （）.
A.Frequency meter B.Ammeter C.Voltmeter D.All of the above
单项选择题 The sub-multiple prefix “nano” (n) means （）.
单项选择题 Possible phase relationships between voltage and current in an alternating current circuit include which of the following conditions?（）
单项选择题 The resistance of a conductor varies （）
