3.“Life expectancy”is the average age at death of the entire live-born population.In the middle of the nineteenth century,life expectancy in North America was 40 years ,whereas now it is nearly 80 years .Thus ,in those days ,people must have been considered old at an age that we now consider the prime of life.
which of the following,if true ,undermines the argument above?
(A) In the middle of the nineteenth century ,the population of North America was significantly smaller than it is today.
(B) Most of the gains in life expentancy in the last 150 years have come from reductions in the number of infants who die in their first year of life
(C) Many of the people who live to an advanced age today do so only because of medical technology that was unknown in the nineteenth century
(D) The proportion of people who die in their seventies is significantly smaller today than is the proportion of people who die in their eighties.
(E) More people in the middle of the nineteenth century engaged regularly in vigorous physical activity than do so today.
我一直搞不懂这句话的解释，谁能告诉我吗？life expectaancy 是什么意思啊？能帮我分析一下这道题目吗？谢谢了
the first question is telling you what Life expectency is. It is an average age at death of someone.
为什么选择b啊？麻烦帮我解释下好吗？
前提（事实）： 19世纪中叶 平均寿命40， 现在 平均寿命80
结论： 现在人活得长了很多：） (原来人觉得已经很老的年龄，在现在的人看来还很年轻)
使一个集合的平均值变大有很多种可能性，有可能是这个集合的个体大多成近似等幅变大趋势；也有可能这个集合中一部分个体激增，使得平均值变大，这个时候方差变小了（也就是说距离平均值特别远的值小了）。
说个极端的例子，如果19世纪中叶有一般人都是1岁死了，一般人是80岁死的，那平均寿命就在40岁左右；而现在1岁的人都80岁死了，平均寿命就变成80岁左右了。
那么这样就不能说原来人觉得已经很老的年龄，在现在的人看来还很年轻，因为原来的时候也有不少人活到了80岁哈。：）
懂了，多谢版主们。
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