http://www.myenglishpages.com/site_php_files/grammar-exercise-past-progressive.php
https://www.ego4u.com/en/cram-up/grammar/past-progressive/exercises?04
https://www.ego4u.com/en/cram-up/grammar/past-progressive/exercises
https://www.perfect-english-grammar.com/past-continuous-exercise-1.html