How to disable remaing radio buttons upon selection of 1 option in 1st attempt

Hi
I am creating a quiz in excel by using option buttons for the answers. Now my question or requirement is:

Once the User selects the answer for a question by clicking on one radio button out of 3/4 options available for that question the selection/answer should get locked. I mean that User should have only one attempt/selction of right/wrong answer for each question.

I.e. I don't want to give user to change his selection of answer for attempted question.

So how to achieve this in excel.....

You could have a look at this site.It has a survey with userforms and radiobuttons

http://www.ozgrid.com/forum/showthread.php?t=64264