You executed the following command to perform a backup of the USERS tablespace：  SQL> ALTER TABLESPACE users BEGIN BACKUP； ALTER TABLESPACE users BEGIN BACKUP *  ERROR at line 1：  ORA-01123： cannot start online backup； media recovery not enabled  What could be the reason for this error（）

• A、The MTTR Advisor is disabled.
• B、The database is in NOARCHIVELOG mode.
• C、The tablespace is already in backup mode.
• D、The Flash Recovery Area is not configured.