A. It repairs blocks that have read disk I/O errors.
B. It checks whether the alias metadata directory and the file directory are linked correctly.
C. It repairs blocks by always reading them from the mirror copy and writing them to the original location.
D. It reads the blocks from a good copy of an ASM mirror and rewrites them to an alternate location on disk if the blocks on the original location cannot be read properly.

A. Only data files and the control file are backed up.
B. Archived redo logs are backed up only if the database is open during the backup.
C. The backup is deleted after one day，regardless of the default retention policy settings. 
D. The backup becomes obsolete after one day，regardless of the default retention policy settings.