Directions:
Read the following text.Choose the best word(s) for each numbered blank and mark A, B, C or D on ANSWER SHEET 1.( 20 points )

Among the devastating consequences of AIDS has _1_ its epidemic spread in the developing world. The disease has caused _2_ suffering, debilitation, loss of life and disruption of family, social and economic _3_. Because of the considerable expense and logistical difficulty in providing antiviral drugs to populations _4_ with the human immunodeficiency virus _5_ the world, the biomedical community is looking towards vaccines to help solve this compelling problem.
The search for an AIDS vaccine began more than 15 years ago with great _6_ and high expectations. With the _7_ of the human immunodeficiency virus (HIV) as the cause of AIDS, it seemed that a _8_ would follow closely behind. But despite a large concerted effort, the problem has proven more difficult than _9_, and progress has not _10_ the _11_ hopes. Here I review the _12_ scientific obstacles confronting the development of an effective HIV vaccine, and I consider _13_ strategies to overcome these obstacles.
It is instructive to consider the circumstances that have _14_ to past successes in vaccine development. The smallpox vaccine is _15_ the most successful inventions in the history of _16_. Why, 200 years ago, without the benefit of modern biotechnology, did the smallpox vaccine succeed so readily while an AIDS vaccine _17_ elusive? The answer lies in an experiment of nature that provided, to an astute observer, a clear direction for smallpox vaccine development. In this classic story of _18_ discovery, Edward Jennet noticed that milk maids who had previously contracted cowpox were _19_ to smallpox infection. This observation was the critical event leading to the finding that the cowpox virus crossª²reacted immunologically with the smallpox virus and could _20_ be used to protect against smallpox.
1. A. on B. with C. been D. about
2. A. unpredicted B. uncontrollable C. unimaginable D. unprecedented
3. A. stability B. instability C. permanency D. soundness
4. A. harmed B. infected C. infectious D. infectable
5. A. everywhere B. throughout C. devastating D. occupying
6. A. difficulty B. concern C. optimism D. pessimism
7. A. confirmation B. identification C. information D. precaution
8. A. cure B. capsule C. medication D. vaccine
9. A. ever B. anticipated C. possible D. necessary
10. A. surpassed B. outª²balanced C. matched D. rivaled
11. A. origin B. initial C. great D. modest
12. A. majority B. primary C. principal D. premier
13. A. potential B. initiative C. practical D. existing
14. A. lead B. caused C. contributed D. cooperated
15. A. by far B. till now C. among D. considered
16. A. vaccine B. medicine C. smallpox D. AIDS
17. A. slowly B. fails C. remains D. counts
18. A. accidental B. importance C. scientific D. vaccine
19. A. vulnerable B. resistant C. opposing D. defendant
20. A. safely B. therefore C. as well D. possibly