单项选择题
An advantage of DC motors over AC motors is that they （）
A.are less expensive B.require less maintenance C.can be started across the line D.offer a more effective means of controlling speed
单项选择题 Which of the following devices are protected from being motorized by a reverse-power relay?（）
单项选择题 If the length of a wire is halved and the cross-sectional area is doubled, the resistance will be （）
单项选择题 AC and DC generators are similar in that they （）
