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 （）
- AThe breaker should be closed when one synchronizing lamp is dark and the other is bright
- BThe breaker should be closed when both synchronizing lamps are bright
- CThe frequency meter should be used to determine that the incoming alternator frequency is slightly higher than the bus
- DA portable phase sequence indicator must be used to verify the information from the lam