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  3. If athletes want better performances, they should train at high altitudes. At higher altitudes, the body has more red blood cell

If athletes want better performances, they should train at high altitudes. At higher altitudes, the body has more red blood cell

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问题 If athletes want better performances, they should train at high altitudes. At higher altitudes, the body has more red blood cells per unit volume of blood than at sea level. The red blood cells transport oxygen, which will improve performance if available in greater amounts. The blood of an athlete who trains at high altitudes will transport more oxygen per unit volume of blood, improving the athlete’s performance. Which of the following, if true, would be most damaging to the argument above, provided that the athlete’s heart rate is the same at high and low altitudes?
选项 A、Scientists have found that an athlete’s heart requires a period of time to adjust to working at high altitudes.
B、Scientists have found that the body’ s total volume of blood declines by as much as 25 percent at high altitudes.
C、Middle-distance runners who train at high altitudes sometimes lose races to middle-distance runners who train at sea level.
D、The performances of athletes in competitions at all altitudes have improved markedly during the past twenty years.
E、At altitudes above 5,500 feet, middle-distance runners often better their sea-level running times by several seconds .
答案B
解析 本题推理可提炼为两句话：“因为每单位血液中红血球多，所以高海拔训练好” 一般地，当段落推理由单位或比例而得出一个结论时，推理成立暗含假设为“总量或数量没有发生变化”、反对主要是反对假设。(B)通过指出血液总量下降而反对了假设，所以(B)正确。(A)中的“require a period oftime”与推理并无直接关系。有人认为若如(A)所说，高海拔训练需适应一段时间，那就相当于指出了高海拔训练的缺点，所以起到了反对作用。但究竟高海拔训练带来的最终收益我们并不知道，完全有可能最终收益大于训练适应成本，所以(A)为无关选项；(C)中的“sometimes”说明不了任何问题；(D)中的“atall altitudes”把范围扩大；(E)中指出了一个“5000英尺”的特例，也并不能对上面描述的一般现象构成反对。
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