The principle we designed around was that it is important to give users feedback for every option: it's important to say to people who got something right why it was right, in case they were right for the wrong reasons, or guessed. In the case of a multiple answer question, each selected answer will display it's feedback when submitted. So you can say;
Which of these were British Primeministers?
Winston Churchill [fb: yes, Winston Churchill was a UK PM]
Tony Blair [fb: yes, Tony Blair was a UK PM]
George W Bush [fb: No, GWB may be many things, but a UK PM is not one of them]
If you haven't found all the right answers yet, you are told 'You have not selected all of the correct answers] so the user can try again.