What can cause an OSPF virtual link not to be established?

What can cause an OSPF virtual link not to be established?

A.
The virtual link transit area is not Area 0.

B.
There is a mismatch of the OSPF network type on the virtual link configuration.

C.
There are DR/BDR election errors on the OSPF virtual link.

D.
The ABRs are using the wrong OSPF router ID in the virtual link configuration.

E.
The virtual link is set up over an NBMA network.