Which set of ordered pairs is a function?

A.
{(2,1), (2,2), (3,4), (5,6)}

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

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

D.
{(3,2), (4,1), (3,5), (1,4)}