Which table represents a function?

Question

x|3|-1|1|-1
y|0|4|2|-2
________

x|0|1|0|2
y|2|3|0|3
_______

x|-2|2|4|-2
y|5|1|5|5
_______

x|5|4|5|1
y|1|5|4|5

I am not sure how to know. So please explain so next time I can know how :).