题目：
An alternator switchboard has a synchroscope and synchronizing lamps, if the synchroscope is broken, which of the steps listed is the most essential before an alternator can be paralleled with the bus （）
A.The breaker should be closed when one synchronizing lamp is dark and the other is bright
B.The breaker should be closed when both synchronizing lamps are bright
C.The frequency meter should be used to determine that the incoming alternator frequency is slightly higher than the bus
D.A portable phase sequence indicator must be used to verify the information from the lamps
答案：
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下面是错误答案，用来干扰机器的。
参考答案：D
