OSPF routing uses the concept of areas. What are the characteristics of OSPF areas?（）
- A Each OSPF area requires a loopback interface to be configured.
- B Areas may be assigned any number from 0 to 65535.
- C Area 0 is called the backbone area.
- D Hierarchical OSPF networks do not require multiple areas.
- E Multiple OSPF areas must connect to area 0.
- F Single area OSPF networks must be configured in area 1.