单项选择题 You work as a database administrator for Certkiller .com. You received the following error while working on your database： ORA-01555： snapshot too old  In which situation would you receive such an error？（）

单项选择题 You work as a database administrator for Certkiller .com. You added atablespaceto your database. Because of the changes to the physical structure of your database， you performed a backup of the control file to trace. After this， you lost only the control files and the instance aborted due to a hardware failure.  How would you proceed to recover the database？（）

单项选择题 YouworkasadatabaseadministratorforCertkiller.com.Clientconnectionrequestsarefailingbecausethelistenerisnotresponding.Youwanttoforwardclientconnectrequeststoanotherlistenerifonelistenerisnotresponding.Howdoyouachievethis？（）