Dynamically changing dropdownmenus

[Icheb]

I have two different dropdownmenus and I would like to change the available choices in menu B upon which entry the user selected in menu A.

For an example the content of menu A is "1" and "2". If the user has selected "1", menu B should contain just "X" and "Y", if the user has selected "2", menu B should contain just "3" and "4".

How is this possible without having to reload the page, maybe in javascript?

[Gamefreak]

You can definitely do it with Javascript, but I don't know how. I'm sure someone else here does, though.