Which set of coordinates describes a function ?
{(-5, -1), (-3, -3), (-1, -5), (-5, -7)}
{(6, 3), (4, 5), (2, 3), (0, 5)}
{(4, -3), (-4, -6), (4, 3), (-4, 6)}
{(2, 4), (2, -4), (4, 8), (6, 2)}