Which set of ordered pairs represents a function?

Question

1) {(1, -2), (2, 4), (2, 7), (-4, -7)}

2) {(1,−2),(2,4),(2,7),(−4,−7)}

3) {(-5, 4), (-1, 7), (-1, -6), (6, -9)}

4) {−5,4),(−1,7),(−1,−6),(6,−9)}

5) {(-1, -2), (9, 8), (-1, -6), (4, 1)}

6) {−1,−2),(9,8),(−1,−6),(4,1}

7) {(0, 4), (7, 0), (2, 3), (5, 0)}

8) {(0,4),(7,0),(2,3),(5,0)}

Answer 1

{ (0,4), (7,0), (2,3), (5,0) }

x value are only allowed to be used once;