In which network environment is IS-IS adjacency check important?

A.
in a multitopology environment where there are different instances of IS-IS running on the same
router

B.
in an IPv4/IPv6 environment and running single-topology IS-IS

C.
when a level L1/L2 IS-IS router is neighboring with a Level 1 only or Level 2 only router

D.
when IS-IS neighbors are in an NBMA environment like over Frame Relay

E.
when IS-IS neighbors are in a broadcast environment like an Ethernet LAN

Explanation: