http://www.english-grammar-lessons.com/pastcontinuous/exercise2.swf
http://oud.digischool.nl/en/grammatica/pastcont-vraag.htm
http://www.focus.olsztyn.pl/en-past-continuous-quiz.html#.VpySPE-vhIM
http://www.english-grammar.at/online_exercises/tenses/past-simple-progressive1.htm
