Which set of ordered pairs represents a function?

{(9,–7), (1, -3), (6, 7), (6, -6)}

{(-7,7), (5,3), (-7, -7),(1, -9)}

{(8,-6), (-9,2), (-9, -8), (-3,4)}

{(-5, -5),(-9,0),(-8,5), (-6,5)}