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

<passage>:
The Olympic Gene Pool Why the human race keeps getting faster. the current record, held by Algerian Noureddine Morceli, is 3:44 , more than 5 percent faster than Bannister's speed. What Iffley Road witnessed was just another step along the road to an ever quicker mile, part of the inexorable improvement of athletic performance that we usually take for granted, particularly when the Olympics roll around. If you stop to think about it, though, such constant progress is remarkable. After all, as biomechanical machines with a standard set of parts, humans should be subject to the same limitations we see in, say, automobiles. How come they aren't? A lot of entrepreneurs and technophiles would like us to think that the answer has to do with discoveries in the world of sports technology. A new Nike shoe is trumpeted as something that will shave at least one-thousandth of a second off your 100-meter time. Trainers measure the rate of buildup of lactic acid in your muscles, then claim that their programs will control it. Nutritionists fine-tune athletes' diets. Even the old sexual-abstinence-before-the-race dogma is being re-evaluated under the all-seeing eye of science. But I consider all this little more than tinkering. Sports records would continue to tumble even if training methods or athletic clothing or sexual practices were exactly the same today as they were in 1896, when the first modern Olympics took place. These minor miracles are the product neither of technology nor of training but of demographic patterns that affect us all. 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. 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 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. You can bring a single generation up to speed through training, but the trends we're dealing with transcend individual generations. Which brings us to another question: Will there come a time when the human machine will hit some sort of natural limit and an Olympic Games pass without a single record tumbling? In principle, yes. There are some barriers that simply cannot be broken. We will never run a mile at the same speed at which we now run 100 meters, for instance. The laws of oxygen exchange will not permit it. Race horses seem already to have hit that outer limit. For years, they were as good as human athletes at pushing back speed records, but then they simply stopped getting faster. Take the prestigious British Derby. From 1850 to 1930, winning times dropped from 2:55 to 2:39. But from 1986 to 1996, the average time has been--2:39. Unlike people, race horses are specifically bred and reared to run. Generations of careful genetic selection have ensured that today's race horse has every possible speed-enhancing characteristic. Training techniques, too, are tremendously sophisticated. But you can go only so far. You can only breed horses with ultralight thin bones to a certain point the bones will break under stress if they get any lighter.

<question>:
The author says, "After all, as biomechanical machines with a standard set of parts, humans should be subject to the same limitations we in, say, automobiles. How come they aren't?" What is a good answer to this question based on the article?

<options>:
A They are subject to the same limits, just not the same quality control.
B Actually, they are subject to biomechanical limitations imposed by factors like the speed at which the lungs can exchange oxygen. It's just that to date, that is not what is capping human performance potential.
C It is specifically untrue that humans have a standard set of parts. There is as much variation in human anatomical details that affect running as in the difference between a Chevy truck and a race car.
D Unlike inorganic automobile parts, the human machine can be improved without replacing any of its parts.

<answer>:

B