Radio buttons should come by pair at the minimum (when dragging a radio to a form).
And they should have an option for one to be checked on by default.
The logic is when you’re using a radio button group, answering it isn’t optional. (ie: book for monday OR tuesday)
If it’s optional, then the whole group should be optional and activated by a checkbox.
Thanks for listening!
Following on this discussion: Radio button - forgetabout it! - #3 by krubens
And about readings such as Radio Buttons: Always Select One?