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)