Which of the following sets of data does not represent a function?

set a
x
y
6
3
8
4
5
7
9
2

set b
x
y
0
3
1
8
2
8
3
-7

set c
x
y
10
7
20
11
30
9
40
7

set d
x
y
3
9
2
2
8
-3
2
1
a.
set a
b.
set b
c.
set c
d.
set d