http://www.nonstopenglish.com/exercise.asp?exid=917
http://www.rong-chang.com/ex/exeasy/lowmodals.htm
http://www.englishexercises.org/makeagame/viewgame.asp?id=2229
http://www.esltower.com/GRAMMARQUIZ/modals/modals%20can't%20have,%20must%20have%20-%20output/modal2.html
http://www.esltower.com/GRAMMARQUIZ/modals/Modals%20of%20probability%20-%20Using%20CAN%20%20and%20%20MUST%20-%20output/modal4.html
http://www.esltower.com/GRAMMARQUIZ/modals/modals%20MUST%20MUSTN'T%20NEEDN'T%20-%20output/modal3.html