1. https://www.ego4u.com/en/cram-up/grammar/passive
2. http://www.perfect-english-grammar.com/passive-exercise-5.html
3. http://www.myenglishpages.com/site_php_files/grammar-exercise-passive.php
4.http://www.learnenglishfeelgood.com/esl-passive-voice-exercise2.html#
