You want to configure the Flashback Database feature and retain flashback logs for three days.  Presently the database is open and configured in ARCHIVELOG mode. The following steps must be performed in the correct sequence to do this：   1. Set the retention target：  SQL> ALTER SYSTEM SET DB_FLASHBACK_RETENTION_TARGET=4320；  2. Enable Flashback Database：  SQL> ALTER DATABASE FLASHBACK ON；  3. Mount the database：  SQL> STARTUP MOUNT EXCLUSIVE  4. Shutdown the instance： SQL> SHUTDOWN IMMEDIATE  5. Open the database：  SQL> ALTER DATABASE OPEN；   Choose the correct sequence.（）

• A、 4， 1， 3， 2， 5
• B、 4， 3， 1， 2， 5
• C、 2， 4， 3， 5， 1
• D、 4， 2， 5， 3， 1