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