Question

Which relation is not a function?

1) {(2,4), (1,2), (0, 0), (-1,2), (-2,4)}
2) {(2,4), (1, 1), (0, 0), (-1, 1), (-2,4)}
3) {(2,2), (1, 1), (0, 0), (-1, 1), (-2,2)}
4) {(2,2), (1, 1), (0, 0), (1,-1), (2,-2)}