Which of the following description of OSPF features is correct?

Which of the following description of OSPF features is correct?
A . The OSPF area boundary is on the link.
B . OSPF is in the broadcast network. All routers form adjacencies.
C . If one LSA must age, any router can advertise LSA aging messages.
D . Generally, all non-backbone areas of OSPF must be connected to the backbone area.

Answer: D