Just for completeness sake, if "no answer" is legitimate and legal, it is better to use check boxes instead of radio buttons. Check box fields with the same name but different return value behave exactly like radio buttons (they represent mutually exclusive items). But they allow the user to uncheck, so that nothing is selected; with radio buttons, this is not possible after the user has made a selection, unless you provide the means to do so (see also clearing marked radio buttons in new form ).
Making the check boxes look like radio buttons is kind of possible; in the Options tab of the field properties set the check box style to "Circle" or "Square".