I'm currently working on the OG for my GMAT prep and I found a question which I answered incorrect and for which I disagree on the official explanation.
The question is the following:
"'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 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 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 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.
How I answered it:
Tough question, because I could not clearly determine the main conclusion/main argument. My first thought had been the following: "People nowadays think that 40 years must be the prime of life, while it actually has been considered old in former times." Answer A would have nothing to do with this, since this argument would not consider population sizes. Answer B would not have much to do with it either because it would disagree with the argument instead of supporting (undermining) it. Answer C does not consider the argument: It just says that people nowadays get older and why. Answer D was my choice (after some thought), because it states that people nowadays on average really are much older (such that the age of 40 would be considered the prime of life). Answer E would not have much to do with the question (it only hints but there is no clear indication of a causal reference between age and action).
How OG answered it:
"What we think of as the prime of life must have been considered old in that era." What point weakens this argument? B would be correct - Greatly reducing first-year infant mortality will have a large impact on the average life expectancy of the population as a whole. That, rather than grown adults living twice as long, is enough to account for a large portion of the doubling in average life expectancy. Answer D.: This point supports the argument.
So I am a little confused now: The answer in my point of view does not fit the question, the question should rather have been "Which, if true, weakens the argument above?" to fit the explanation.
Can anyone explain it to me in his/her own words?
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MGMAT 1 - 580 - Q36 V33
MGMAT 2 - 650 - Q44 V35
GMAT 1 - 690 - Q48 V38
MGMAT 3 - 680 - Q47 V36
MGMAT 4 - 690 - Q44 V39
GMAT 2 - 700 - Q48 V37