Which set of ordered pairs does not represent a function?
{(8,-2), (6,-1), (4,1), (0,1)}
{(8,4), (1,5), (-7,7), (9,5)}
{(4,-6), (-3,2), (7,-1), (-6, -4)}
{(-9,3), (8,7), (-9, -9), (2, 4)}