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

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

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Answer: B