单项选择题

You work as a database administrator for Supportcenter.cn. You received the following error while working on your database： ORA-01555： snapshot too old ，In which situation would you receive such an error？（）

A.You performed rollback after commit.
B.If a log switch occurs in your database.
C.Thearchiverfails to generate an archived log file.
D.Instance recovery fails to find one of the redo log files.
E.A long-running query is unable to get read-consistent image.
F.If you perform manual undo management operations in auto mode.

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单项选择题YouworkasadatabaseadministratorforSupportcenter.cn.Youaddedatablespacetoyourdatabase.Becauseofthechangestothephysicalstructureofyourdatabase，youperformedabackupofthecontrolfiletotrace.Afterthis，youlostonlythecontrolfilesandtheinstanceabortedduetoahardwarefailure.Howwouldyouproceedtorecoverthedatabase？（）

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多项选择题YouworkasadatabaseadministratorforSupportcenter.cn.YourdatabaseisintheMOUNTstateandyouexecutethefollowingcommandtoopenitALTERDATABASEOPEN；Whattwoactionsareperformedasaresultofthiscommand？（）

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