A. BFD was set up incorrectly on Switch-2, so it caused Switch-2 to lose adjacency with Switch-1 rather than repair the session.

B. Graceful restart helper was not enabled on Switvh-2, so BFD was unable to operate correctly, and the session was taken down.

C. A link between Switch-1 and Switch-2 went down. BFD detected the lost connectivity and behaved as expected.

D. A hardware issue caused a unidirectional link; BFD detected the issue at Layer 2 and prevented a broadcast storm.