Which relations are functions and which are not functions?

Hello!

In a function, each input has one output. In 1, three has two outputs, 7 and 6, so it is not a function.. In 3, 4 has two outputs, 2 and 6, so it is also not a function. 2 and 4 all have different inputs, so they are both functions.

B 1
A 2
B 3
A 4

I hope this helps!