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
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A.Alternators
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单项选择题
If the length of a wire is halved and the cross-sectional area is doubled, the resistance will be （）
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AC and DC generators are similar in that they （）
A.both generate alternating voltages
B.both rectify the voltage before delivery
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