Which set of ordered pairs represents a function?

A. {(5,8), (-8, 2), (-1, -2),(-1, -6)}
B. {(-3,6),(-2,5), (6,2), (-3,7)}
C. {(-2,-2),(-9, -2), (7,-6), (-7, -5)}
D. {(-4,-4),(-8, -1),(4,5), (4,-4)}