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.