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