PLZ HELP ME WITH THIS!!!!!!

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