Which two statements are true regarding undo tablespaces（）

A.The database can have more than one undo tablespace.

B.The UNDO_TABLESPACE parameter is valid in both automatic andmanualundo management.

C.Undo segments automatically grow and shrink as needed， acting as circular storage buffer for their assigned transactions.

D.An undotablespaceis automatically created if the UNDO_TABLESPACEparameter is not set and the UNDO_MANAGEMENT parameter is set to AUTO during the database instance start up.