Which are the correct steps， in order， to deal with the loss of an online redo log if the database has not yet crashed？（）   a. Issue a checkpoint.  b. Shut down the database.  c. Issue an alter database open command to open the database.  d. Startup mount the database.  e. Issue an alter database clear logfile command.  f. Recover all database datafiles.

• A、a， b， c， d
• B、b， d， e， c
• C、a， b， d， e， c
• D、b， f， d， f， c
• E、b， d， a， c