That's the idea of a radio button, which can be formatted with a checkmark. Checkboxes are generally used when you can choose more than one; radio buttons for selecting only one.
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The problem with radio-buttons is that the selection can't be undone, unless you reset the form (which it turns out isn't even available in the Reader interface, so it has to be done through a button in the form itself, if the author created it). That's not the case with check-boxes, which is why it's usually better to use the latter.
That's a good point, Gilad. You could, technically, create a reset button that would only reset that set of radio buttons but that would probably add more confusion for the user and I don't especially recommend it.
I hadn't thought of using the check box feature with the radio button function, but it's a great idea. I can't wait until it's time to create my next form so I can try it!