Which of the following relations is a function?
a. (4,4), (-3, 2), (8, 1), (-8, 2)
b. (8, 1), (-3, 4), (4, 1), (8, 2)
c. (4,4), (-3, 6), (4, 3), (-8, 2)
d. (4,0), (-3, 3), (8, 1), (-3, 5)