- http://www.hueber.de/shared/uebungen/schritte-international/fset.php?Volume=2&Lection=2&Exercise=1&SubExercise=1
- http://www.hueber.de/shared/uebungen/planet/lerner/uebungen/fset.php?Volume=1&Lection=13&Exercise=5&SubExercise=1&ExerciseUrl=pl1L13ex5.htm&SubExerciseCount=1