GATE | GATE-CS-2014-(Set-3) | Question 48
Consider the decision problem 2CNFSAT defined as follows:
(B) solvable in polynomial time by reduction to directed graph reachability.
(C) solvable in constant time since any input instance is satisfiable.
(D) NP-hard, but not NP-complete.
Explanation: 2CNF-SAT can be reduced to strongly connected components problem. And strongly connected component has a polynomial time solution. Therefore 2CNF-SAT is polynomial time solvable. See https://en.wikipedia.org/wiki/2-satisfiability#Strongly_connected_components for details.