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？（）
- Aassign users to consumer groups
- Bset the resource plan for the instance
- Cconfigure a simple resource plan and specify plan directives
- Dconfigure a complex resource plan and specify plan directive