Why is the %_COMPLETE refreshed to 13.59 in the third output after reaching 88.77?

View the Exhibit and examine the output of the query in different times when the following command runs in an RMAN sessions:
RMAN> BACKUP DATABASE FILESPERSET 2;
The database has seven data files. Why is the %_COMPLETE refreshed to 13.59 in the third output after reaching 88.77?

A.
Because the progress is reported for each data file

B.
Because the progress is reported for each backup set

C.
Because other RMAN sessions have issued the same BACKUP command

D.
Because new data files have been added to the database while the RMAN backup is in progress

Explanation:
http://download.oracle.com/docs/cd/B19306_01/backup.102/b14191/advmaint.htm#i1008347
While the RMAN job is running, start SQL*Plus and connect to the target database, and execute the longops script to check the progress of the RMAN job.

If you repeat the query while the RMAN job progresses, then you see output such as the following:

SQL> @longops
SID        SERIAL#    CONTEXT    SOFAR      TOTALWORK  %_COMPLETE
———- ———- ———- ———- ———- ———-
8          19         1          10377      36617      28.34

SQL> @longops
SID        SERIAL#    CONTEXT    SOFAR      TOTALWORK  %_COMPLETE
———- ———- ———- ———- ———- ———-
8          19         1          21513      36617      58.75

SQL> @longops
SID        SERIAL#    CONTEXT    SOFAR      TOTALWORK  %_COMPLETE
———- ———- ———- ———- ———- ———-
8          19         1          29641      36617      80.95

If you run the longops script at intervals of two minutes or more and the %_COMPLETE column does not increase, then RMAN is encountering a problem.

Refer to “Monitoring RMAN Interaction with the Media Manager” on page 23-8 to obtain more information.

If you frequently monitor the execution of long-running tasks, then you could create a shell script or batch file under your host operating system that runs SQL*Plus to execute this query repeatedly.