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. 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. 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 it is the accidental byproduct of combining two previously isolated stocks. There are a number of theories to account for this at the genetic level, but it has proved difficult to discriminate among them. It is possible that modern humans exhibit some form of hybrid vigor simply because migration and admixture of populations are now occurring at unprecedented rates. Perhaps, just perhaps, such hybridization is being translated into enhanced performance. That doesn't mean, however, that genetic differences in athletic ability can be correlated automatically with race. That is a claim that is impossible to test, because you cannot control, in an experimental sense, environmental differences among the study groups. Sure, you will find more Africans or descendants of Africans standing on the podiums at the end of Olympic track events. And you will find far fewer Asians on those same podiums. But can you, therefore, conclude that Africans have better genes for running than Asians do? No. Environmental differences between the two groups could account for differing levels of athletic success. It is scarcely surprising that Ethiopian or Kenyan distance runners do better than everyone else, since they are in the habit of running immense distances to and from primary school, middle school, and high school. The training is what's crucial, not the blackness. The Chinese sports establishment also has carried out an enormous, and effective, experiment to help dispel the myth that race has a direct relation to athletic ability. Until recently, a quick glance at the medals table confirmed every stereotype people held about Asians and sports. Then the Chinese decided to produce record-breaking female distance runners (and swimmers), and, boy, did they ever. In 1992, China ranked fourth in the Olympic-medal haul. 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. Human improvement, like race-horse improvement, must eventually bow to the basic constraints of biomechanics. The age of menarche cannot keep on falling forever. On the other hand, it is clear from the remarkable demographic changes of just the past 20 years that these long-term trends are with us still. They may be slowing down in some more developed societies, but they roar along in others. And these trends will continue to fuel the improvement in athletic performance. Several new records will be set in Atlanta. And in Sydney in 2000, and wherever the Olympics are held in 2044. We will continue running faster and jumping further for a good long while to come.

<question>:
What genetic influences on running speed does the author identify?

<options>:
A The author is focused on differences in "nurture" and doesn't believe that there are any examples of differences in "nature" (genes).
B Body type matters. If you are not born an ectomorph, a genetically controlled body type, you will not be able to run fast over long distances.
C He calls out abnormal genetic conditions that would impede speed, and also references the effect of mixing races in producing "hybrid vigor."
D Secretariat had an abnormally large heart. As with horses, people born with genetically larger hearts can run faster and longer.

<answer>:

C