Which set of ordered pairs represents a function?
O {(7,4), (3,8),(-4,-4),(0,4)}
O {(-5,1),(3, -7), (2,-4), (2,7)}
O {(-8,0), (-5,6), (-8,1),(-6,5)}
O {(4,2),(4, -9),(-9,-6), (7,1)}