7. Which of the following relations is a function?
A.{(2, 3), (-2, 3), (3, 2), (-3,-2)}
B.{(8,-4), (-4, 8), (-4, -8), (-8,4)}
C.{(9, -1), (-1, 9), (9, 2), (2, -1)}
D.{(5, -7), (4, 6), (-3, 8), (5,9)}