Select all relations that represent functions.
(8,7), (7,8), (6,5), (5,6)
(-2,7), (-3,9), (-5,9), (-8,11)
(6,-6), (-3,12), (6,-1), (-8,13)

x y
-3 0
-5 8
-7 8
-9 0

x y
0 0.5
2 1
1 2.5
0 2