Which relation is not a function?
a) { (1, -5), (3, 1), (-5, 4), (4, -2) }
b) { (1, -5), (-1, 6), (1, 5), (6, -3) }
c) { (2, 7), (3, 7), (4, 7), (5, 8) }
d) { (3, -2), (5, -6), (7, 7), (8, 8) }