Which relation is a function?


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

{(4, 2), (3, 3), (2, 4), (3, 2)}

{(1, −1), (−2, 2), (−1, 2), (1, −2)}

{(1, 2), (2, 3), (3, 2), (2, 1)}