input: Text: The Olympic Gene Pool Why the human race keeps getting faster. By Andrew Berry ( 2,168 words; posted Thursday, July 4; to be composted Thursday, July 11 ) 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-
  Question: What practicial limit did Thoroughbreds bump into which has help stalled the speed gains they made during the 19th and early 20th centuries?
  
   (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:
output: 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.