21. A. known B. clear C. obvious D. sure

22. A. would B. must C. might D. should

23. A. had B. ought C. used D. wanted

24. A. bicycle B. hill C. straw D. worker

25. A. when B. before C. while D. as

26. A. thinking B. suspecting C. expecting D. insisting

27. A. thoroughly B. through C. down D. up

28. A. carefully B. successfully C. obviously D. silently

29. A. return B. retreat C. retire D. rest

30. A. short while B. a period C. few minutes D. some time