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Click on the 'None' response in the box on the left, then in the Response options in the centre of the screen, click on 'Must be the only response' and 'Reject Category'.  This indicates that no other answers must be present in a question where multiple responses are allowed, and is the answer to be selected if no other responses are given by the respondent.  Click 'OK' when finished, then 'Save & New' to start the next question.