Which set of ordered pairs could NOT represent a function?

a.) (1, 1), (2, 2), (5, 5), (7, 7), (13, 13)
b.) (1, 5), (2, 6), (3, 6), (4, 5), (5, 4)
c.) (2, 3), (3, 4), (2, 5), (5, 6), (2, 7)
d.) (3, 2), (2, 1), (4, 3), (5, 4), (6, 0)