(Help asap!) Select Is a Function or Is not a Function to correctly classify each relation.

which one is a function and which is not?
{(3,2),(6,4),(9,6),(12,8)}
{(2,3),(2,5),(3,6),(9,4)}
{(2,4),(6,3),(4,8),(6,6)}
{(3,4),(5,2),(6,8),(6,4)}