21. He failed in ________ experiment, but he wanted to have ______ second try.
A. an; the B. the; an C. an; a D. the; a
22. ---Did you listen to the news?
---No, I _______ some clothes then.
A. was washing B. washed C. had washed D. was going to wash
23. In this class, there should be no such students _______ often break the rules.
A. that B. who C. as D. they
