1-3 BAC
4-7 DCAD
8-11 ABDC
12-15 ADAB
16. B 17. A 18. G 19. F 20. D
21. D 22. A 23. C 24. A 25. D
26. D 27. B 28. A 29. C 30. C
31. B 32. D 33. B 34. A 35. D
36. D 37. C 38. A 39. C 40. B
41. was built 42. in 43. to walk 44. spending 45. better
46. hired 47. but 48. the 49. watchtowers 50. daily
短文改错：
1.are→is
2.go后加to
3.friend→friends
4.删除turn后的out
5.little→few
6.our→their
7.talk→talking
8.kept→keep
9.where→when/whenever
10.mostly→most
