Which of the following lists of ordered pairs is a function?
A. (1,1),(2, 2), (3, 1), (3, 2)
B. (-2, 3), (2, 4), (-3,6), (3,7)
C. (0,3), (-1,4),(1,6), (-1,7)
D. (2,5), (3,-4), (4,7), (2, 2)