Which relation below is a function?

a)
x y
0 2
1 3
0 4
2 5

b)
x y
0 0
1 1
0 8
2 27

c)
x y
0 -2
1 -3
0 -4
2 -5

d)
x y
0 0
1 -4
2 8
3 4