Which of the following relations is not a function?

A. { (0,1), (1,1), (2,1), (3,1)}
B. { (0,0), (3,4), (5,6), (8,9)}
C. { (0,1), (0,2), (0,3), 0,4)}
D. { (0,0), (2,2), (4,4), (6,6)}