You need to ensure that the database users should be able to use the various flashback query features in order to go back in time by four hours. What would you do to achieve this?（）

• A、set SQL_TRACE=True 
• B、set UNDO_RETENTION=14400 
• C、set FAST_START_MTTR_TARGET=240 
• D、set LOG_CHECKPOINT_INTERVAL=240 
• E、set DB_FLASHBACK_RETENTION_TARGET=14400 
• F、set the RETENTION GUARANTEE clause for the undo tablespace