单项选择题

The most common source of excitation for synchronous motors is a/an （）.

A.low voltage battery
B.motor attenuator set
C.DC exciter generator
D.AC supply

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Before reassembling any machinery, you should （）.

A.replace all bearings regardless of length of service
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Fuses placed in series with a thermal trip-type circuit breaker are used for （）.

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