单项选择题
Possible phase relationships between voltage and current in an alternating current circuit include which of the following conditions?（）
A.Current and voltage may be in phase B.Current may lead the voltage C.Current may lag the voltage D.All of the above
单项选择题 The resistance of a conductor varies （）
单项选择题 A switchboard for an AC electrical system requires the use of which of the following devices（）.
单项选择题 The part of the shipboard electrical system used to control the distribution of power to the branch circuits, is the （）.
