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

B.
{ (-3, -4), (-4, -3), (3, 4), (4, 3) }

C.
{ (3, 3), (3, 4), (4, 3), (4, 4) }

D.
{ (0, 0), (0, 3), (3, 0), (3, 3) }