Which relation(s) is/are not a function?

A (-4, 6), (0, 6), (7, 6), (4, 6), (-3, 6)
B (6,-4), (6, 0), (6, 7), (6, 4), (6, -3)
C (4, 1), (8, 2), (12, 3), (16, 4), (20, 4)
D (1, 4), (2, 8), (3, 12), (4, 16), (4, 20)