You are working on a 24X7 database. You want to design a backup strategy for your database that uses user managed backups. You want to be able to perform all backups while the database remains online.  Which statement about performing user-managed backups in a 24x7 environment is true？（）

• A、 You must have change tracking enabled in your database
• B、 Your database must be running in NOARCHIVELOG mode
• C、 To back up a tablespace，it must be in backup mode
• D、 To back up a tablespace，it must first be taken offline