In this case, which is the most appropriate method of treatment?

An enterprise U1900 series unified gateway deployment of disaster recovery, one day burst communication failure, causes all users cannot make a voice call. Check found that failure is due to the master device downtime, while the standby device is not prepared to take over the business result.

In this case, which is the most appropriate method of treatment?
A . first check whether the standby equipment is normal, try to manually switch to resume operations
B . to locate the cause of not master-slave switching after the main equipment failure
C . to locate the cause of the main equipment downtime

Answer: A