在Oracle数据库中，关于何时一个回滚段会由ONLINE变为OFFLINE的描述中，哪些是错误的？ 当使用该回滚段的所用事务都已提交并过期后 当UNDO表空间不足时，会考虑将ONLINE的回滚段变为OFFLINE，以便随后将OFFLINE的回滚段占用的空间释放。 当使用该回滚段的会话从数据库上断开或结束后 How to Keep All UNDO Segments from Being Offlined in Oracle 10g - Fast Ramp-Up (Doc ID 396863.1)中提及“ When are UNDO segments OFFLINED? Answer: SMON decides on the # of undo segs to offline and drop based on the max transaction concurrency over a 12 hour period in 9i. This behavior is altered in 10g where the max concurrency is maintained over a 7-day period. Moreover, in 10g SMON doesn't drop the extra undo segs, but simply offlines them. ” 执行以下命令在数据库服务器中创建密码文件： $ orapwd file = orapworcl entries = 5 ignorecase=N 哪个语句描述了上述密码文件的用途？D A、录所有添加到OSDBA或OSOPER操作组的用户的用户名和密码。 B、它包含启用了审核的用户的用户名和密码 C、当授予DBA角色时，它记录用户的用户名和密码 D、Oracle使用它为远程数据库管理员认证用户