http://michel.barbot.pagesperso-orange.fr/cap/yes_no/y_n1.htm
http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-7790.php
http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-6350.php
http://www.montsemorales.com/gramatica/BeorHavegot1.htm