og11th 3 请帮助解答.谢谢

3, "Life expectancy" is the average age ate 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 neraly 80 years. Thus, in those days, people must have been considered old at an age the we now consider the prime of life.

which of the following, if true, 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.

各位,偶刚开始看逻辑,就被这道题弄糊涂了,我实在没有看出逻辑关系所在,请麻烦解释一下.

OG上得解释是:B was falesly assumed that age for an entire population was simply extended when actually the average age ate time of death was significantly raised when the number of infants dying in their first year was reduced.

"Life expectancy" is just the average number of years people live as a whole before they die, it is a combination of both people who die at young age and people who age and die gradually.

B points out the real reason why now the average death age is older -- with less people die when they are infants, the average age at death as a whole will become a lot older.

Hence, B weakens the argument and is the correct answer.