Which relation is also a function?
{(-1, 3), (0, 2), (3, -1), (3, 2)} {(2,-4), (2,-2), (2, 1), (2, 3)}
{(0, 3), (1, 2), (1, 4), (2.1)}
{(-4,-3), (0.-3). (1, -3), (4, -3)}