13) The baby ______ (sleep) when the dog suddenly ______ (start) barking. 14) I ______ (meet) my cousin while I ______ (shop) at the mall. 15) They ______ (wait) for the train when they ______ (hear) the announcement.

1. The baby was sleeping when the dog suddenly started barking.
2. I met my cousin while I was shopping at the mall.
3. They were waiting for the train when they heard the announcement.

Past Continuous обозначает длительное действие, происходившее в момент другого действия; Past Simple — краткое завершённое действие.