2. 外语
  3. Among the devastating consequences of AIDS has 【21】______ its epidemic spread in the developing world. The disease has cause

Among the devastating consequences of AIDS has 【21】______ its epidemic spread in the developing world. The disease has cause

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问题       Among the devastating consequences of AIDS has  【21】______   its epidemic spread in the developing world.  The disease has caused  【22】______   suffering, debilitation,  loss of life and disruption of family, social and economic  【23】______   Because of the considerable expense and logistical difficulty in providing antiviral drugs to populations  【24】______   with the Human Immunodeficiency Virus  【25】______   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  【26】______   and high expectations. With the  【27】______   of the Human Immunodeficiency Virus (HIV) as the cause of AIDS, it seemed that a  【28】______   would follow closely behind. But despite a large concerted effort, the problem has proven more difficult than  【29】______   , and progress has not  【30】______   the  【31】______   hopes. Here I review the  【32】______   scientific obstacles confronting the development of an effective HIV vaccine, and I consider  【33】______   strategies to overcome these obstacles.
     It is instructive to consider the circumstances that have  【34】______   to past successes in vaccine development. The smallpox vaccine is  【35】______   the most successful inventions in the history of  【36】______   Why, 200 years ago, without the benefit of modern biotechnology, did the smallpox vaccine succeed so readily while an AIDS vaccine  【37】______   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  【38】______   discovery, Edward Jennet noticed that milk maids who had previously contracted cowpox were  【39】______   to smallpox infection. This observation was the critical event leading to the finding that the cowpox virus cross-reacted immunologically with the smallpox virus and could  【40】______   be used to protect against smallpox.
【40】
选项 A、safely
B、therefore
C、as well
D、possibly
答案B
解析 根据上下文可知，空格前是因果关系，故应填therefore表所以。
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