Which set of ordered pairs is not a functions?

A. (3,1), (2,1), (1,2), (3,2)
B. (0,0), (1,1), (2,2), (3,3)
C. (4,1), (5,1), (6,1), (7,1)
D. (1,2), (3,4), (4,5), (5,6)