Quiz: Blood Cousins

Blood Cousins: Count nodes at distance exactly $k$ from node $v$ that share the same $k$-th ancestor. What's the core insight?

1. A.Sort all nodes by depth
2. B.Use Floyd-Warshall
3. C.Find the $k$-th ancestor, then count descendants at depth $k$ below it
4. D.Use BFS from each query node