Read the following passage and questions, then choose the right answer from options, the answer should be one of A, B, C, D.

<passage>:
Over the past century, the human race has been affected by a slew of what demographers call "secular" trends. (In this context, "secular" does not refer to a trend's lack of spirituality but to its longevity: Secular trends are long-term modifications, not just brief fluctuations.) One such trend is an increase in average size. You have to stoop to get through the doorways of a Tudor cottage in England because its inhabitants were smaller than you are, not because they had a penchant for crouching. Another trend is in life expectancy. People are living longer. Life expectancy in Africa increased over the past 20 years from 46 to 53 years. Over the same period in Europe, where things were already pretty comfortable to begin with, life expectancy increased from 71 to 75 years. The global average was an increase from 58 to 65 years. Probably the most striking change, though, is how much more quickly children are maturing. A 12-year-old child in 1990 who was in what the World Health Organization calls "average economic circumstances" was about 9 inches taller than his or her 1900 counterpart. This is not solely the product of the first trend--the increase in average size--but also due to the fact that children develop faster. Girls menstruate earlier than they used to. The age of menarche (the onset of menstruation) has decreased by three or four months per decade in average sections of Western European populations for the past 150 years. There is a good chance that our 1990 12-year-old already had started to menstruate. Her 1900 counterpart would still have had three years to wait. What do such trends have to do with athletic performance? Well, if we're living longer and growing up faster, that must mean we're producing bigger, better bodies. Better bodies imply faster miles. We run faster and faster for the same reason it is now common for 11-year-old girls to menstruate. But why are these things happening? Demographers have offered a variety of explanations, but the main one is that our diet is improving. A 12-year-old ate better in 1990 than she would have in the Victorian era. This conclusion is supported by studies of the social elite: Because its members were well-nourished even in the early years of this century, this group has experienced relatively little change, over the past 100 years, in the age girls first menstruate. Another explanation is that health care is getting better. In 1991, according to the WHO, more than 75 percent of all 1-year-olds worldwide were immunized against a range of common diseases. Smallpox, that scourge of previous generations, now is effectively extinct. Probably the best measure of how much healthier we are is the rate of infant mortality, which measures both the health of the mother (a sickly mother is more likely to produce a sickly baby) and the health of the baby. In the past 20 years, infant mortality around the world has dropped from 92 deaths per 1000 live births to just 62. A lot of this can be chalked up to primary-heath-care programs in the developing world--the African average, for instance, has dropped from 135 deaths per 1000 births to 95. But there are also significant improvements in the developed world, with infant deaths dropping in Europe over the same 20-year period from 24 per 1000 live births to just 10. Better health care affects athletic ability directly. This is true in the trivial case in which, say, antibiotics cure a runner's fever before the big race, but it may also be true in a more significant way. Diseases contracted in early infancy can have a lifetime impact on health--not necessarily a big one, but an impact nevertheless. Previous generations bore scars from all sorts of non-life-threatening diseases, the stuff everyone picked up as a baby. Nowadays, though, more and more people grow up with no history of disease. Since top athletes inevitably are drawn from the healthiest sector of the population, a generally superior system of health care means a bigger pool of people to draw from. You are much more likely to find someone who can run a mile in 3:30 in a sample of several million superbly healthy people than you are in a sample of 10,000. The pool of potential athletes has expanded in other ways, too. First, the population has exploded. Second, we are coming ever closer to a worldwide middle class, the class from which athletes typically are drawn. Whether, in an age of multinational capitalism, we may talk reasonably about a post-colonial era is way beyond the scope of this article. The fact remains, however, that the developing world is doing just that--developing. Even Mozambique, which ranks at, or near, the bottom of national per capita gross national product tables, has shown an increase of some 20 percent in adult literacy rates over the past 20 years. Literacy rates are merely an index of education, which itself is another way of talking about a global move away from a hand-to-mouth lifestyle. The decline of empire has its Olympic corollaries. Britain won, on average, 17 gold medals per Olympics in the five official games held in its imperial heyday before World War I. That average has dropped to only five medals per Olympics in the 17 held since. This is not a reflection of declining athletic standards in Britain, however it's a function of how much more competitive other nations have become. The Olympics originally were the preserve of the socioeconomic elite of the socioeconomic elite among nations. Consider this: Only 13 nations participated in 1896, but there were 172 in 1992. Black Africans didn't take part until the third modern games, held in St. Louis in 1908. Even this was accidental: Lentauw and Yamasami, Zulu tribesmen, entered the marathon because they happened to be in St. Louis as part of an exhibit about the Boer war. Lentauw finished ninth despite being chased into a cornfield by dogs. Since all these are changes in how we live, not anything innate, we have to conclude that what we are describing here are effects of environment, not genes. Let us assume that our 1900 and 1990 12-year-olds are identical twins magically born 90 years apart. The 1990 girl still will grow up faster, end up bigger, menstruate earlier, and live longer than the 1900 girl. Perhaps way, way back in human history, when our forebears were still fleeing saber-toothed tigers, natural selection for athletic prowess came into play. But all that ended long ago. Indeed, the laws of natural selection probably work against athletes these days: Given the rigors of training schedules, it is possible that today's top athletes have fewer children than average.

<question>:
According to the author, the age of onset of girls' periods is an indicator of improved diet, one factor in the improved health conditions correlated with humans running faster. That being the case, what group might be expected not to have had much improvement in their athletic performance in the last hundred or so years, based on what happened to their age of onset of periods during that time?

<options>:
A The upper crust of society, people who already and always had enough money to remain well-fed, and therefore already performed better, and did not stand to gain as the general level of nutrition improved.
B People who experienced cycles of good and bad fortune would have only benefited a little from society's improved health conditions.
C People of average economic circumstances have continued to have average economic circumstances, therefore their health did not improve, and athletes from this social stratum have not improved.
D Girls were not allowed to compete in athletic events at the time, so no one really knows whether improving nutrition helped their speed.

<answer>:

A