Which set of ordered pairs does NOT represent y as a function of x?
a {(9, 6), (10, 6), (11, 6), (12, 6)}
b {(1, 0), (2, 3), (4, 5), (2, 6)}
c {(4, 5), (5, 6), (6, 7), (7, 8)}
d {(1, 10), (2, 5), (-1, 5), (-2, 10)}