Rewrite these sentences without changing their meanings

VI. Rewrite these sentences without changing their meanings. (1.0 point)
21. We couldn’t join the festival because we were very busy.
 We were very ………………………………………………………………….………………..
22. If she doesn’t try harder, she can’t pass the exam.
 Unless…. …………………………………………………………….…….…………………..
23. During my dinner, the phone rang.
 While I …….. ……………………………………………………………..……………………
24. There are lots of festivals in Viet Nam during the year.
 Viet Nam ………………………………………………………………………………………..