Which of the following statements is a contradiction?


a.)
(p ∧ ¬p) → ¬(q ∨ ¬q)


b.)
(p ∧ ¬p) ↔ (q ∨ ¬q)


c.)
(p ∧ ¬p) ↔ ¬(q ∨ ¬q)


d.)
¬(p ∧ q) → ¬(p ∨ q)