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