Which set of ordered pairs represents a function?

A. {(1, 7), (8, -9), (1, -2), (-6, -1)\}{(1,7),(8,−9),(1,−2),(−6,−1)}
B. {(9, 0), (7, 9), (3, -4), (3, -6)\}{(9,0),(7,9),(3,−4),(3,−6)}
C. {(-2, -6), (-5, -6), (9, -2), (7, 3)\}{(−2,−6),(−5,−6),(9,−2),(7,3)}
D. {(4, 7), (8, 1), (-6, -1), (-6, -5)\}{(4,7),(8,1),(−6,−1),(−6,−5)}