What is the correct order of steps to perform an online database backup？（）   a. alter database begin backup b. alter database end backup c. Back up the database datafiles d. Back up the archive log files e. alter system switch logfile

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