Unchecking option boxes (newbie question)

[Narwe]

I have an option box with the options 'Yes' and 'No'. How do I make them uncheckable? That is, if someone makes a mistake and clicks either yes or know when neither of them should have been clicked, how can I permit them to uncheck their selection and leave nothing selected?

 

[smercer]

I have an option box with the options 'Yes' and 'No'. How do I make them uncheckable? That is, if someone makes a mistake and clicks either yes or know when neither of them should have been clicked, how can I permit them to uncheck their selection and leave nothing selected?

Have a third option called "None of the above"