You want the ability to recovery any time within the last seven days and therefore you configured the recovery window retention policy using the command：   RMAN> CONFIGURE RETENTIOH POLICY TO RKCOVFRY WINDOW OF 7 DAYS；   After configuring the recovery window， you performed the database backup as follows：（）  

• A、 Backup RB1 at log sequence number 12871 on 5th Jan
• B、 Backup RB2 at log sequence number 15622 on 12th Jan
• C、 Backup RB3 at log sequence 16721 on 15th Jan
• D、 On 20th Jan when the log sequence number was 18112 you realize that there Is a need to a point in time at the beginning of the recovery window. You have all an hived redo log files to date.