You performed an incremental level 0 backup of a database: RMAN > BACKUP INCREMENTAL LEVEL 0 DATABASE; To enable block change tracking after the incremental level 0 backup, you issued this command: SQL > ALTER DATABASE ENABLE BLOCK CHANGE TRACKING USING FILE ‘ /mydir/rman_change_track.f’; To perform an incremental level 1 cumulative backup, you issued this command: RMAN> BACKUP INCREMENTAL LEVEL 1 CUMULATIVE DATABASE; Which three statements are true?（）
- A Backup change tracking will sometimes reduce I/O performed during cumulative incremental backups.
- B The change tracking file must always be backed up when you perform a full database backup.
- C Block change tracking will always reduce I/O performed during cumulative incremental backups.
- D More than one database block may be read by an incremental backup for a change made to a single block.
- E The incremental level 1 backup that immediately follows the enabling of block change tracking will not read the change tracking file to discover changed blocks.