Which relation is a function?

A. {(1, −1), (−2, 2), (−1,  2), (1, −2)}

B. {(1, 4), (2, 3), (3,  2), (4, 1)}

C. {(1, 2), (2, 3), (3,  2), (2, 1)} ​

D. {(4, 2), (3, 3), (2,  4), (3, 2)}