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.