2. 外语
  3. Before 1970, children in the United States were commonly vaccinated against smallpox. In the early 1970s, large-scale vaccinatio

Before 1970, children in the United States were commonly vaccinated against smallpox. In the early 1970s, large-scale vaccinatio

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问题 Before 1970, children in the United States were commonly vaccinated against smallpox. In the early 1970s, large-scale vaccinations of children against smallpox were discontinued because smallpox had virtually disappeared from the world. Recently, some scientists have cautioned that the Unite States could be exposed to smallpox again. Contrary to what many people believe, this exposure would probably find all age-groups within the United States population at comparable levels of risk of contracting smallpox, since______.
Which of the following most logically completes the passage?
选项 A、smallpox is a highly communicable disease among people who are not immune to it
B、smallpox vaccine is not now widely available in the United States
C、the population of the United States has been growing more slowly since 1970 than it was before then
D、smallpox vaccinations lose their efficacy after about 20 years
E、people in the United States travel more than they did in 1970, thereby increasing the chances that an epidemic could spread
答案D
解析 A．smallpox is a highly communicable disease among people who are not immune to it天花在对它没免疫能力的人当中特别容易传播
解析：本文的矛盾之处在于接种了疫苗的人还会感染天花，而该选项讨论的是本身就对天花没有免疫的人。无关，不选。
B．smallpox vaccine is not nOW widely available in the United States天花疫苗现在在美国不容易获得
解析：本文讨论的关键点是之前就接种过疫苗的人，跟现在疫苗容不容易得到没有关系。不选。
C．the population of the United States has been growing more slowly since 1970 than it was before then 1970年之后美国人口增长的速度比之前更慢
解析：与人口增长速度无关。不选。
D．smallpox vaccinations lose their efficacy after about 20 years天花疫苗在20年后会失效
解析：天花疫苗在20年后失效了，因此1970年接种过疫苗的人现在也有可能感染天花。正确。
E．people in the United States travel more than they did in 1970，thereby increasing the chances that an epidemic could spread现在的美国人相比于1970年旅游更多，因此增加了瘟疫爆发的可能性。
解析：旅游的增加提高了瘟疫爆发的可能性，依然无法解释为什么接种了疫苗还会感染天花。不选。
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