GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - YAEW256BG7W89T15J

Which of the following most logically completes the passage? 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