OG--3 ''Life expectancy''is the average at death of the entire live-born population. In the middle of the nineteenth century, life expectancy in the 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 ture , 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 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 because of medical techolog 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 od 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. 请各位帮我分析一下它问什么选B，谢谢！ | 
