单项选择题
If the length of a wire is halved and the cross-sectional area is doubled, the resistance will be （）
A.quartered B.unchanged C.doubled D.quadrupled
单项选择题 AC and DC generators are similar in that they （）
单项选择题 To increase the frequency of an operating AC generator, you should （）
单项选择题 When measuring AC current flow, you must always connect the meter （）
