Which of the following is a function?
A. (-2,3), (0,3), (1,6), (1,-6)
B. (-2,4), (0,3), (1,6), (-1,2)
C. (1,3), (2,4), (1,5), (2,3)
D. (1,-1), (2,-2), (1,-3), (4,-4)