please help

Which of the following relations describes a function?
A.
{ (0, 0), (1, 1), (4, 2), (9, 3) }

B.
{ (0, 0), (1, 1), (1, -1), (4, 2) }

C.
{ (-4, 2), (-1, -1), (-1, 1), (0, 0) }

D.
{ (0, 0), (1, 1), (2, -2), (2, 2) }