请教og11-3

“life expectancy” is the average age at death of the entire live-born population.In the middle of the nineteenth centry,life expectancy in North America was 40 years,whereas now it  is nearly 80 years.Tus ,in those days,people must have been considered old at an age that we now consider the prime of life.

which of the folllowing,if ture,undermines the argument above?

b. most of the gains in life expectancy 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 beause of medical technology that ws 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.

答案选择b

我今天刚刚开始逻辑题，问题可能比较白痴，但是我看了og解释以后还是不明白。婴儿成活率高了，人的平均寿命不久高了么？希望有好心人帮我解答。

但是，要推翻的结论不是“平均寿命提高”，而是“40这个年龄，在现在被认为是中年，而在过去被认为是老年”。

简单的说，就是在19世纪，40岁这个年龄也不能被划为是“老年”，之所以那个时候的平均寿命是40，是因为科技不发达，婴儿的死亡率高。

可是这样的话，c不是也不对吗？