Which three statements are true?

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.

Explanation:
A: In a cumulative level 1 backup, RMAN backs up all the blocks used since the

most recent level 0 incremental backup.
E: Oracle Block Change Tracking
Once enabled; this new 10g feature records the modified since last backup and stores the log of it
in a block change tracking file using the CTW (Change Tracking Writer) process. During backups
RMAN uses the log file to identify the specific blocks that must be backed up. This improves
RMAN’s performance as it does not have to scan whole datafiles to detect changed blocks.
Logging of changed blocks is performed by the CTRW process which is also responsible for
writing data to the block change tracking file.
Note:
* An incremental level 0 backup backs up all blocks that have ever been in use in this database.