which condition will the L1-L2 router set the attach bi…

An L1 router has formed an adjacency with an L1-L2 router.
Under which condition will the L1-L2 router set the attach bit in its Level 1 link-state PDUs?

A.
when the L1-L2 router forms an adjacency with another L1 router in the same area

B.
when the L1-L2 router forms an adjacency with an L2 router in the same area

C.
when the L1-L2 router forms an adjacency with an L2 router in a different area

D.
when the L1-L2 router forms an adjacency with an L1 router in a different area

Explanation:
L1 routers use L1/L2 routers as the next hop for the default route 0.0.0.0/0. The L1/L2 routers do not advertise
a default route—instead, the L1/L2 routers set the attached bit, which is advertised to the L1 router and causes
the L1 router to install a default route with the L1/L2 router as the next hop.
If an L1/L2 router does not have an L2 adjacency to a different area than its own, the L1/L2 router will not set
the attach bit, resulting in the L1 routers not installing a default route.
Network Scaling With Bgp Labeled Unicast, Design and Configuration Guide, page 8