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  3. Ethiopians appear to have evolved a unique way of coping with thin mountain air. But how they do it remains a mystery. One wa

Ethiopians appear to have evolved a unique way of coping with thin mountain air. But how they do it remains a mystery. One wa

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问题    Ethiopians appear to have evolved a unique way of coping with thin mountain air. But how they do it remains a mystery.
   One way for the body to get enough oxygen to its tissues when breathing oxygen-poor air is for it to make more red blood cells. This increases the amount of hemoglobin(血红蛋白), the protein that carries oxygen. Although less haemoglobin in the arteries is saturated with oxygen at high altitudes, having more of it makes up for the shortfall.
   People native to the high Andes are known to have more red blood cells than lowlanders, and athletes who train at altitude can increase their concentration of cells.
   But while many Tibetans also live at high altitudes, they do not have significantly elevated levels of haemoglobin. Instead they seem to boost the amount of nitric oxide, which dilates(使膨胀) blood vessels and increases blood flow.
   Now Cynthia Beall, an anthropologist from Case Western Reserve University in Cleveland, Ohio, has found a third kind of adaptation. When she tested the blood of 236 people in the Ambaras region in the Semien Mountains of Ethiopia, she found that 95 percent of the haemoglobin in their arteries is saturated with oxygen, almost as much as that of people living at low altitudes and roughly 5 per cent above that of residents in the Andes or Tibet.
   "That shouldn’t be, "says Beall. They must have a massively efficient way to get oxygen from the lungs to the blood, she says. But just what remains mysterious. They do not have higher concentrations of haemoglobin than anyone else, nor do they have a different kind of haemoglobin.
   Beall adds that this ability might be found in all people living in that part of the world, and not just those in the study. It might be why so many world-class endurance athletes are Ethiopian. "The next study needs to look at that,"she says.
From the passage we can infer that the faster our blood flows, ______.
选项 A、the stronger we become
B、the weaker we become
C、the less nitric oxide we have
D、the lower levels of haemoglobin we have
答案D
解析  文章的第4段讲到西藏人也居住在高海拔地区，但他们的血红蛋白并不高，却有很高含量的氧化氮，氧化氮使得血管膨胀，血液流动加速。作者在这里暗示血液流动加速会降低血红蛋白的含量，而人们在快速奔跑时血液流速肯定会加快，所以该题的答案应该是D。
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