Which two OSPF network scenarios require OSPF virtual link configuration?

A.	to connect an OSPF non-backbone area to area 0 through another non-backbone area
B.	to connect an NSSA area to an external routing domain
C.	to connect two parts of a partitioned backbone area through a non-backbone area
D.	to enable route leaking from Level 2 into Level 1
E.	to enable route leaking from Level 1 into Level 2
F.	to enable OSPF traffic engineering