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