- http://www.eflnet.com/grammar/relpronoun.php#.VCIQvtJDt1Y
- http://www.ego4u.com/en/cram-up/grammar/relative-clauses/exercises?08
- http://www.esltower.com/GRAMMARQUIZ/intermediate/relative%20pronoun.htm
- http://www.ego4u.com/en/cram-up/grammar/relative-clauses/exercises?02
- http://www.esl-lounge.com/student/grammar/3g31-relative-clause-gap-fill-exercise.php