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