http://www.englishexercises.org/makeagame/viewgame.asp?id=5794
http://www.englishexercises.org/makeagame/viewgame.asp?id=3634
http://www.englishexercises.org/makeagame/viewgame.asp?id=1255
http://www.perfect-english-grammar.com/relative-clauses-exercise-1.html
http://www.perfect-english-grammar.com/relative-clauses-exercise-2.html