30.In which situation may the UNDO_RETENTION parameter be ignored, even if it is set to a value?

30.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 tablespaces 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
答案：C
解析：UNDO_RETENTION生效的首要条件是undo表空间的retention为guarantee，因此AB排除
这里undo tablespace是不是确定的我感觉其实意义不大，但是下边的文档中有这么一段，所以选择C
参考：http://docs.oracle.com/cd/E11882_01/server.112/e40402/initparams265.htm#REFRN10225
UNDO_RETENTION specifies (in seconds) the low threshold value of undo retention. 
For AUTOEXTEND undo tablespaces, the system retains undo for at least the time specified in this parameter, 
and automatically tunes the undo retention period to satisfy the undo requirements of the queries. 
For fixed- size undo tablespaces, the system automatically tunes for the maximum possible undo retention period, 
based on undo tablespace size and usage history, and ignores UNDO_RETENTION unless retention guarantee is enabled.