PRACTICE:
https://www.allthingsgrammar.com/uploads/2/3/2/9/23290220/atg-quiz-relclauses.pdf
http://baladre.info/english/sedaviwebfront/relative6.htm
https://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-1951.php
http://www.english-4u.de/relative_pronouns_ex4.htm
https://www.ego4u.com/en/cram-up/grammar/relative-clauses/exercises?09
http://www.englishexercises.org/makeagame/viewgame.asp?id=4219
