the s-genitive

http://www.englisch-hilfen.de/en/exercises/nouns_articles/apostrophe_s2.htm
http://www.englisch-hilfen.de/en/exercises/nouns_articles/apostrophe_s.htm
http://www.tolearnenglish.com/exercises/exercise-english-1/exercise-english-135.php
http://www.englishexercises.org/makeagame/viewgame.asp?id=2452
http://first-english.org/english_learning/english_beginners/s_genitive_exercises/01_s_genitive_exercises.htm
http://www.learning-english-online.net/grammar/parts-of-speech-and-sentence-structure/nouns/possessive/s-genitive/
http://first-english.org/english_learning/english_beginners/s_genitive_exercises/02_genitive_s_exercises.htm

s-genitive or plural-s?
http://first-english.org/english_learning/english_beginners/s_genitive_exercises/03_s_genitive_rules_exercises.htm
http://first-english.org/english_learning/english_beginners/s_genitive_exercises/04_s_genitive_or_plural_s_exercise.htm
http://first-english.org/english_learning/english_beginners/s_genitive_exercises/05_s_genitive_or_plural_s_exercises.htm