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Which two statements are true regarding undo tablespaces? (Choose two.)
A.The database can have more than one undo tablespace.
B.The UNDO_TABLESPACE parameter is valid in both automatic and manual undo management.
C.Undo segments automatically grow and shrink as needed, acting as circular storage buffer for their
assigned transactions.
D.An undo tablespace is automatically created if the UNDO_TABLESPACE parameter is not set and the
UNDO_MANAGEMENT parameter is set to AUTO during the database instance start up.

A 正确，可以有一个或多个 undo 表空间，但同一时间只能使用一个 undo 表空间
B 错误，官方文档明确指出只有在自动 undo 管理模式，UNDO_TABLESPACE 才有效
C 正确，undo 段会根据需要自动增加和收缩
D 错误，undo 表空间只能 DBCA 自动创建或者手动创建，不会自动生成