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單項(xiàng)選擇題

The magnetic field is provided by electromagnets so arranged that adjacent poles have （）.

A.opposite polarity
B.the same polarity
C.negative polarity
D.positive polarity

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單項(xiàng)選擇題

An a-cswitchboard is usually（） the corresponding d-cboard because of the higher voltage and lower currents and smaller bus-bars.

A.larger and heavier than
B.as much as
C.smaller and lighter than
D.more dangerous than

單項(xiàng)選擇題

The star formation is most commonly used and requires（） on the alternator.

A.one slip-ring
B.two sliprings
C.three sliprings
D.four sliprings

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