Field based on other field

Hello,

How can I set a field based on another.
If I have a Select1 field with option 1,2,3, I want that in Select2 field to have option 1a and 1b if I select option 1 and 2 from Select1, or 1j if I select option 3 from Select1.

http://wordpress.org/extend/plugins/contact-form-7-dynamic-text-extension/