When performing an online backup，what is the proper order of the following steps？（）   a. Issue the alter database end backup command. b. Back up the archived redo logs. c. Issue the alter database begin backup command.  d. Back up the database files.  e. Determine the beginning log sequence number. f. Determine the ending log sequence number.  g. Force a log switch with the alter system switch logfile command.

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