WPF, XML databinding into dependent/cascading ComboBoxes

Question

I have an XML file with the following structure:

Answer 1

Okay, since I found an answer to my more specific question, I think I know the answer to this question, too. I don't need [Serializable] classes for the different nodes in my XML, because I can just use XAML and XPath to create cascading/dependent ComboBoxes: