Visible the field

I want to make field invisisble and visible on change of two option set . There are two options 1)Yes 2) No

and also there are 2 option set Graduation and PostGraduation. If I select yes then only graduation should be there ,Postgraduation should be invisible and vice versa. If I say No then its working fine for first time . but again if m saying YES then then it dosent give Graduation visisble. How to do it???