Which relation is a function?

A
(1, 1), (2, 2), (3, 3), (4, 4), (5, 8)

B
(2, 7), (6, 5), (4, 4), (3, 3), (2, 1)

C
(9, -3), (9, 3), (4, -2), (4, 2), (0, 0)

D
(1, 0), (3, 0), (1, 1), (3, 1), (1, 3)