Explanation/Reference:
A: Fast Incremental backups is possible with Block change tracking, which is initially introduced from version 10.2 onwards, by this tool it's very useful to reduce the RMAN incremental backup duration.
If the changes are something around 20% then in this situation BCT helps a lot.
C: Exadata Storage Server offload capability combined with RMAN block change tracking will efficiently perform large I/Os at the storage-tier level, returning only individual changed blocks for incremental backups and increasing the backup performance of the system.
Note: Level 1 backup: A level 1 backup includes only those blocks that have been changed since the
"parent" backup was taken. Remember a parent backup may be either a level 0 or a level 1 backup.
Block change tracking allows indeed the highest benefit for databases where the changes are not so high, Level 0 backup: A level 0 incremental backup is physically identical to a full backup and it includes every data block in the file except empty blocks. The only difference is that the level 0 backup is recorded as an incremental backup in the RMAN repository, so it can be used as the parent for a level 1 backup.
References: http://www.dba-oracle.com/t_rman_backup_types.htm
http://www.oracle.com/technetwork/database/availability/maa-tech-wp-sundbm-backup-11202-183503.pdf
https://www.toadworld.com/platforms/oracle/w/wiki/11124.fast-incremental-backups-active-data-guard