A system must be upgraded from AIX 5.1 to 5.2.  Which of the following scheduling processes should be performed? （）

• A、 Schedule enough down time to backup, upgrade and reboot. 
• B、 Backup, upgrade, then schedule enough down time to reboot the system. 
• C、 Backup, then schedule enough down time to upgrade and reboot the system. 
• D、 No scheduling processes are necessary because no down time is required.