Which of the following represents a function?

a.
x -10 -8 -3 -3
y 16 14 16 15

b.
x -10 -8 -10 3
y 16 16 22 15

c.
x -10 -8 -3 3
y 16 14 22 16

d.
x -10 -8 -3 -8
y 16 14 16 15