Which relation is a function?
{(1,2), (2, 1), (1, 3), (3,1)}
{(1,2), (3, 4), (5, 6), (1,8)}
​ {(0,2), (0, 4), (0, 6), (0,8)}
{(1,2), (2, 3), (3, 4), (4,5)}