Identify the relation that is also a function.
a.
{(5, 4) (5, 1) (5, 3) (0, 5)}
b.
{(0, -1) (1, -2) (2, -3) (-1, 2)}
c.
{(1, 0) (2, 3) (3, -1) (1, 5)}
d.
{(4, 2) (4, -3) (9, 3) (9, -3)}