Which set of ordered pairs does not represent a function?
{(5,7), (4,-2), (-8,8), (4,-9)}
{(6,-2), (-2, -7), (-9,-4), (8,-7)}
{(-6,-7), (0,9), (6,-7), (3, -4)}
{(4,6), (9,-8), (3,2), (-9,-8)}