Which set of values is a function?

a.
(6,-5) (7, -3) (8, -1) (9, 1)

b.
(9,5) (10,5) (9,-5) (10,-5)

c.
(3,4) (4,-3) (7,4) (3, 8)

d.
(2, -2) (5, 9) (5, -7) (1, 4)