You have created a resource plan， DB_PLAN， using DBMS_RESOURCE_MANAGER.CREATE_SIMPLE_PLAN and you want to impose a restriction on utilization of resources by the database users. Which step must be performed before you can start using the resource plan？（）

A .  assign users to consumer groups B .  set the resource plan for the instance C .  configure a simple resource plan and specify plan directives D .  configure a complex resource plan and specify plan directive