You performed the RMAN database backup having a backupset key number 231 with the KEEP FOREVER option.  After some days， you want to change the status of the database backup and you issued the following command：   RMAN>CHANGE BACKUPSET 231 NOKEEP；   What is the implication of this command？（）

• A、 The backup is deleted.
• B、 The backup is marked unavailable.
• C、 The backup overrides the backup retention policy.
• D、 the backup becomes eligible for deletion according to the existing retention policy.