The list below displays four steps that you need to execute in order to switch from the primary database role to the standby database role. Choose the correct order of execution for the steps.（）  1. Shot down and start up the former primary instance without mounting the instance.  2. Issue the ALTER DATABASE COMMIT TO SWITCHOVER TO PHYSICAL  STANDBY command.  3. Issue the ALTER DATABASE MOUNTS STANDBY DATABASE command.  4. End read or update activity on the former primary and standby database.

• A、1，2，3，4
• B、3，2，4，1
• C、4，2，1，3
• D、4，3，2，1