You are maintaining your database in Oracle10g. You set the value of the UNDO_RETENTION initialization parameter to zero in the initialization parameter file. What will be the impact of specifying this setting？（）  

A. The database will not start.
B. The database will start but will not retain the undo data in the undo segment.
C. The database will start， and the undo segment will contain the undo data for at least one hour.
D. The database will start， and the undo segment will retain the undo data for at least 15 minutes.