单项选择题
A.RECOVER LOFIEL BLOCK GROUP 2；
B.ALTER DATABASE DROP LOGFILE GROUP 2；
C.ALTER DATABASE CLEAR LOGFILE GROUP 2；
D.ALTER DATABASE RECOVER LOGFILE GROUP 2；
E.ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP 2；
单项选择题 YouworkasadatabaseadministratorforCertkiller.com.Inyourdatabaseserver，theparameterPLSQL_OPTIMIZE_LEVELhasbeensetto2.Whatwouldthissettingachieve？（）
单项选择题 You work as a database administrator for Certkiller .com. You find that reports are running for a long time in your data warehousing applications. You suspect a lack of indexes is the reason for the performance degradation. Which advisory component would you refer to， in order to determine the appropriate indexes？（）
单项选择题 Two database users， Jack and Bill， are accessing the Certkiller STAFF table of the Certkiller DB database. When Jack modifies a value in the table， the new value is invisible to Bill. Which is the modified value invisible to Bill？（）