Which set of ordered pairs represents a function?

a. {(8,-5), (1,0), (1, -5), (-5,6)}
b. {(9,--4), (5, 4), (9, -6), (4, - 7)}
c. {(7,-8), (-2, -6), (-1,-1), (-5, -1)}
d. {(-3, -3), (8,6), (8,7), (1,8)}