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  3. Life Span About a century ago, the average life span for Americans was about 50 years. /Today, the typical American lives fo

Life Span About a century ago, the average life span for Americans was about 50 years. /Today, the typical American lives fo

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答案 Life Span About a century ago, the average life span for Americans was about 50 years. /Today, the typical American lives for around seventy-eight years. / What accounts for such increased longevity? /Between 1900 and 1950, inventions such as refrigeration and sewage treatment meant that/young people were able to survive longer. /Moreover, medical breakthroughs helped contain diseases/such as some fatal ones that killed many children. /These advances helped increase the average life span. /This means that future medical breakthroughs will result in even longer average life span.
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