Which of the following is not a function?
A{ (0,1), (1,2), (2,3), (3,4)}
B.{(0,2), (1,3), (4,3), (1,2)}
C.{(1,3), (4,2), (2,0), (3,4)}
D.{(1,2), (2,2), (3,2), (4,2)}