A.Back up the Flash Recovery Area.
B.Decrease the retention time for the database backup and flashback log files.
C.Manually delete all the archived log files from the Flash Recovery Area by using operating system （OS）commands.
D.Manually delete all the expired backup sets from the Flash Recovery Area by using operating system（OS） commands.

A.The backup piece is expired as the new backup is available.
B.The backup piece is expired because the operating-system file was not found.
C.The backup piece is expired because the retention period for the backup has expired.
D.The backup piece is expired because the backup set to which it belongs is not complete.