Which of the following are characteristics of OSPF areas?（）
- A Hierarchical OSPF networks need to be in one area
- B Multiple OSPF areas must connect to area 0
- C Single area OSPF networks must be configured in area 1
- D Areas can be assigned any number from 0 to 63535
- E Area 0 is called the backbone area
- F Each OSPF area need to be configured with a loopback interface