PLEASE HELP


Which of the following sets of ordered pairs represents a function?
A.
{ (0, -6), (11, -6), (0, -6), (-6, 11) }
B.
{ (-6, 6), (-6, 9), (-6, 0), (9, -6) }
C.
{ (6, 11), (6, 1), (0, 0), (8, 13) }
D.
{ (6, -6), (9, -6), (0, -6), (-6, 9) }