Which set of ordered pairs represents a function?

1-{(−5,−4),(3,4),(7,−6),(7,1)}

2-{(8,−2),(6,8),(6,3),(−4,−2)}

3-{(7,5),(7,−9),(−2,−3),(4,−1)}

4-{(−8,−7),(0,−7),(−4,0),(4,−5)}