Which relations are functions?

x
y
-2
-4
-2 -6
0
-8
2
-10

x
y
0
4
10 5
20
6
10
7

x
y
2
-10
1 -20
0
-10
-1
-20

x
y
2.5
0
3.5 4
4.5
8
6.5
12

x
y
-10
4
-20 5
0
6
-10
7
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