Conditional Form Fields

I need a form that will replace our site Contact Us form.  The current Contact Us form has conditional fields.  For instance, they select "Need CAD Model/Drawing.  When this is selected, fields called Part Number, CAD File Format, CAD File Version are added to the form.

I can also use a Page 2 of the Contact Us form if the above is not possible.  Once Page 1 is complete and "Need CAD Model/Drawing" is selected, Page 2 of the form would pop up and include the fields Part Number, CAD File Format, CAD File Version would appear.