Select the sets that are not functions. e = {(6, 2), (7, 3), (6, -1), (5, 4)} a = {(1, 2), (2, 3), (3, 4), (4, 5)} c = {(-1, 3), (0, 3), (1, 3), (2, 3)} b = {(1, 2), (2, 1), (3, 0), (4, -1)} d = {(1, 1), (1, 2), (1, 3), (1, 4)}