21. Now more and more people have come to know the part that trees _____in our natural environment.
A. do B. take C. play D. work
22. It is so nice to hear from her. ______, we last met more than thirty years ago.
A. What’s more B. That is to say
C. In other words D. Believe it or not
23. When he was out of ____ work, he was involved in crimes of violence and was put in _______prison at last.
A. the; the B. \; \ C. a; \ D. \; the
