Adding an option if one is selected

This one should be easy but I can not figure it out.

I have a checkbox that ask the user if they want a certain option. It is not checked by default. But if they check it I would like another dropdown to show up so they can select what type of upgrade they want (only options doesn't effect price)

But I also need to have another checkbox pop up after they do that that will allow them to add another upgrade....it will be the same as the first--default will be no but if they pick yes I need that dropdown menu again.

Help--thanks

Tim,
You might place a post in the commercial section for this type of attribute..