Which set of ordered pairs represents a function?

{(5, -8), (8,-1), (8,7), (-4,2)}

{(-9,-9), (7, -9), (-7, -7), (9,-8)}

{(4,7), (-7,-4), (6,8), (-7,6)}

{(0,8), (-7,2), (4, -9), (4, -3)}