Drop Down Questions

Drop Down Questions

I want to create a form for registration for a website.

The problem is I want the answer to question 3 to be a drop down selection of answers (not a problem) . 
However i then want question 4 to have a drop down selection of answers. (not a problem)

the problem is i want there to be a different choice of answers to question 4 depending on the answer the user chose for question 3. 

PLEASE HELP