Quiz: Debugging 2-SAT

Your $2$-SAT solution reports "unsatisfiable" but you believe a solution exists. You have $n$ variables and check scc[i] != scc[i + n] for each variable $i$. What is the most likely bug?

1. A.Forgetting to run Tarjan's algorithm
2. B.Not sorting the SCC numbers
3. C.Running BFS instead of DFS for SCC
4. D.Using $i + 1$ instead of $i + n$ for the negation node, causing wrong variable-negation pairing