Last one (for now)
Which of the following relations describes a function?
A.
{ (-1, -2), (-2, -1), (1, 2), (2, 1) }

B.
{ (1, 1), (1, 2), (2, 1), (2, 2) }

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

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