Which relation is a function?


A.
{(1, 2); (2, 3); (3, 4); (2, 5)}


B.
{(1, 2); (2, 3); (3, 4); (4, 5)}


C.
{(1, 2); (1, 3); (1, 4); (1, 5)}


D.
{(1, 2); (3, 2); (3, 3); (4, 2)}