答案:
1. A. Besides
2. C. Connected
3. B. host
4. A. avoid
5. D. involving
6. A. on
7. D. exposed
8. C. down
9. D. calculated
10. C. explained
11. D. Even
12. B. symptoms
13. B. increased
14. C. associated
15. B. generate
16. C. in the face of
17. A. attribute
18. B. because
19. A. remains
20. D. influences