Which of these tables represents a function?

a)
game number season 1 season 2
1 3980 3210
2 4060 4180
3 4190 3020
4 3820 3670
5 4100 5320

b)
x f(x)
0 -1
1 0
2 -1
1 -2

c)
x f(x)
0 2
1 2
2 2
3 4

d)
x f(x)
1 -2
0 -1
1 -2
4 1