Which of the following relations is not a function?
A. (3, 4), (-2, 2), (3, 1), (-7, 2)

B. (-7, 4), (3, 3), (-2, 1), (7, 2)

C. (-2, 4), (3, 2), (7, 1), (-7, 5)

D. (7, 4), (-2, 2), (3, 1), (-7, 2)