Which two are true about offloading backups to a physic…

Which two are true about offloading backups to a physical standby database in a Data Guard environment?

A.
The standby database must be registered in an RMAN catalog after the primary database has been
registered.

B.
The standby database cannot be registered in an RMAN catalog if the primary database has not been
registered.

C.
Backups of the standby control file taken while connected to the catalog where the database is registered,
may be used to restore the control file on the primary database.

D.
The standby database must be registered in an RMAN catalog before the primary database has been
registered.

Explanation:
Only the primary database must be explicitly registered using the REGISTER DATABASE command. You do
this after connecting RMAN to the recovery catalog and primary database as TARGET.
https://docs.oracle.com/database/121/SBYDB/rman.htm#SBYDB4851