Which set of ordered pairs represents a function?

\{(-1, -1), (-7, -5), (5, -4), (5, -2)\}{(−1,−1),(−7,−5),(5,−4),(5,−2)}
\{(-9, 9), (-8, 1), (2, -4), (-2, 1)\}{(−9,9),(−8,1),(2,−4),(−2,1)}
\{(-6, 3), (-9, 5), (0, -1), (-9, 6)\}{(−6,3),(−9,5),(0,−1),(−9,6)}
\{(5, 8), (-5, 7), (5, 0), (-7, -9)\}{(5,8),(−5,7),(5,0),(−7,−9)}