GIVING BRAINLIST

Which set of ordered pair represents a function?

A. { (1,2), (2,3), (3,4), (4,5) }

B.{ (-5, 2), (-6,5), (-7, 9), (-8, 10), (-5, 11) }

C.{ (1,3), (2,4), (5,6), (2,8) }

D.{ (4,9), (5,8), (8,6), (4,7) }