Which of the following is not a function?
A. {(4,5), (4,2), (5,4), (2,4)}
B. {(1,2), (2,3), (3,4), (4,5)}
C. {(1,2), (2,3), (3,3), (4,5)}
D. {(4,1), (5,1), (6,1), (7,1)}