Which set of ordered pairs does not represent a function?

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