Which relation is a function? a. {(1, −1), (−2, 2), (−1,  2), (1, −2)} b. {(1, 4), (2, 3), (3,  2), (4, 1)} c. {(1, 2), (2, 3), (3,  2), (2, 1)} d. {(4, 2), (3, 3), (2,  4), (3, 2)}