What does the DB_FLASHBACK_RETENTION_TARGET parameter configure？（）  

• A、 An upper limit on how far you can flash back the database，depending on the information in the redo logs
• B、 An upper limit on how far you can flash back the database，depending on the information in the undo tablespace
• C、 The amount of time for which the flashback data is to be kept in the flash recovery area， provided that there is enough space
• D、 The amount of time for which the flashback data is guaranteed to be kept in the undo tablespace，provided that there is enough space