Fifty years from now the world’s population will be declining, with no end in【B1】______.Several centuries from now, unless peopl

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问题     Fifty years from now the world’s population will be declining, with no end in【B1】______.Several centuries from now, unless people’s values changes【B2】______, there could be fewer people living in the entire world than live in the United States today. The big【B3】______of the past twenty years is that in not one country did fertility stop falling when it reached the【B4】______rate -- 2.1 children per women. In Italy, for example, the rate has【B5】______to 1.2. In Western Europe as a whole and in Japan it is down to 1.5. The evidence now【B6】______that within fifty years or so world population will【B7】______at about eight billion before starting a【B8】______rapid decline. That has really posed a great deal of anxiety.【B9】______, many people still fear that it will keep "exploding" until there are too many people for the earth to support.  But【B10】______.  World population was growing by two percent a year in the 1960s; the rate is now down to one percent a year, and【B11】______.This view is coming to be widely accepted among population experts, even as the public continues to focus on the threat of uncontrolled population growth.
【B4】

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答案replacement

解析 名词,这句话的意思是出生率达到某一水平后继续下降就会出现问题,这肯定是保持人口基数的水平,即replacement rate。  
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