clear add variable when user changes answ

I have six questions on a page, On each dropdown box I have set it up so that the answer the user selects' value (which is numeric) will add that value to a score variable.So, if they select the answer with a value of 2 the score variable is then 2. The problem that I just realized is if the user selects 2 then changes there mind and select 5 the total added to the score is 7. One way around this I guess would be to have only one question on the page and make the add to variable(score) on the next button rather than the select of the dropdown box. However I was hoping I could avoid that. I can't think of a way around this besides the one I just gave, if anyone can help it would be greatly appreciated. Thanks

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