Your multitenant container database (CDB) contains a pluggable database, HR_PDB. The default permanent tablespace in HR_PDB is USERDATA. The container database (CDB) is open and you connect RMAN. You want to issue the following RMAN command: RMAN > BACKUP TABLESPACE hr_pdb:userdata; Which task should you perform before issuing the command?（）

• A、Place the root container in ARHCHIVELOG mode.
• B、Take the user data tablespace offline.
• C、Place the root container in the nomount stage.
• D、Ensure that HR_PDB is open.