AUM has been retaining about 15 minutes worth of undo. You want to double the retentionperiod， but not at the expense of new transactions failing. You decide to alter the system to set the parameter UNDO_RETENTION=18000. However， AUM still retains only about 15 minutes worth of undo.  What is the problem？ （） 

• A、 You need to alter the undo tablespace to add the RETENTION GUARANTEE setting.
• B、 You need to increase the size of the undo tablespace.
• C、 The undo tablespace is not set to auto-extend.
• D、 You need to alter the Recycle Bin to add the RETENTION GUARANTEE setting.