Which set of ordered pairs represents a function?
{(9,8), (-1,7),(-1, –9), (3, -1)}
{(9,-5), (4,6), (9,8),(-6,9)}
{(-8,0), (-8,2), (5,–7), (-2, -9)}
{(2,9), (-4,-6), (9,6), (0,6)}​