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