Which of the relationships below is a function?
a. (6,3), (5,2), (6,8),(0,7)
b. (8,2), (1,7), (-1,2), (1,9)
c. (4,3), (3,0), (-1,3), (2,7)
d. (7,1), (0,0), (6,2), (0,4)