You have a production instance running on your server. UNDO_RETENTION is not long enough to satisfy read-consistency requirements.
How do you change the UNDO_RETENTION value?
A. by executing the ALTER SYSTEM ... command
B. by re-creating the control file with a new value for UNDO_RETENTION
C. by executing the ALTER DATABASE ... command in the MOUNT state
D. by executing UPDATE ... statement on V$PARAMETER to change the value of UNDO_RETENTION
Answer: A