A. You can write on both primary and standby databases because DB system standby databases in Oracle Cloud Infrastructure Database are logical standbys.

B. You can terminate a primary database DB system.

C. You can use the reinstate command to return a failed database into standby role after correcting the cause of failure.

D. You can switch over the primary database to the standby role, perform maintenance on the standby database, and then switch it back to the primary role.

E. If the primary database becomes unavailable, then you can use Oracle Data Guard to fail over the standby database to the primary role.