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

<passage>:
On May 6, 1954, at Oxford University's Iffley Road track, Roger Bannister became, by just half a second, the first man to run a mile in less than four minutes. The Holy Grail of middle-distance running was his. Forty-two years later, however, that achievement seems less significant. Four-minute miles are commonplace 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. 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? 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 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. Just because nurture has a more significant effect on athletic performance doesn't mean that nature lies dormant, though. Genetic variation exists for just about any trait you choose to study, and the ability to run quickly would be no exception. To take a trivial case, we know that the inheritance of extra fingers or toes is determined genetically. It is quite possible that the possession of an extra toe would hinder an aspiring miler--their genes have affected their athletic performance. One genetic factor that may be influencing performance trends is what is known as "hybrid vigor." Cattle breeders have known about this for a long time: Take two inbred lines of cattle, cross them, and what you have is "better" (say, larger) than any single individual in either of the two parental lines. This does not require natural selection 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

<question>:
What practicial limit did Thoroughbreds bump into which has help stalled the speed gains they made during the 19th and early 20th centuries?

<options>:
A Creating horses that were strong but lightly built ran into trouble at the point when the horses bones were so fragile that a lot of horses started breaking down during races.
B The limits of oxygen change were reached, as proved by a series of very clever experiments involving a Thoroughbred and a treadmill.
C Horses could not get enough oxygen in and out of their lungs, which caused them to bleed. Performance improvement stopped when Lasix was banned in Britain.
D The entire population of Thoroughbreds traces back to just three stallions - The Godolphin Arabian, Byerly Turk and the Darley Arabian. The horses are so inbred now that they have a multitude of genetic problems that drag down their performance.

<answer>:

A