Which set of ordered pairs represents a function? {(-2, 4), (-1, 6), (-6, 3), (-6, 4)} {(5, 5), (9, 5), (-8, 0), (-7, 8)} {(-2, 6), (8, -3), (-8, 4), (8, -8)} {(-2, -1), (-1, 6), (2, 4), (-2, -9)}