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單項(xiàng)選擇題

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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單項(xiàng)選擇題

Which of the following devices are protected from being motorized by a reverse-power relay?（）

A.Alternators
B.Wave guides
C.Exciters
D.Amplidynes

單項(xiàng)選擇題

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

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