In which situation may the UNDO_RETENTION parameter be ignored, even if it is set to a value?
A. When the data file of the undo tablespace is autoextensible
B. When there are more than one undo tablespace available in the database
C. When the undo tablespace is of a fixed size and retention guarantee is not enabled
D. When the undo tablespace is autoextensible and retention guarantee is not enabled
此题考的是 UNDO_RETENTION 即使设置了值，在哪些情况下会忽略。
在固定大小的还原表空间，且没有设置 retention guarantee 强制保留，UNDO_RETENTION 就会被忽略。如果设置了 retention guarantee ，即使是固定大小的表空间，也不会忽略 UNDO_RETENTION，但有可能出现事务失败。