Which of the following relations shows a function?
A.{(5, -7), (6, -7), (-8, -1), (0, -1)}
B.{(4, -1), (4, -2), (3, -1), (2, 4)}
C.{(4, 5), (3, -2), (-2, 5), (4, 7)}
D.{(1, 4), (4, 1), (1, -4), (-4, 1)}