Which of the following relations represents a function?
{(4,2), (4,3), (4, 5), (4,7)}
{(-2, 1), (0,5), (1, -3), (-2, 0)}
{(-1, 3) (-2, 3) (-1, 5) (-2,5)}
{(-3, 1), (1, 4), (-2, 1), (0, 3)}