Which of the following most logically completes the argument given ?

Asthma, a chronic breathing disorder, is significantly more common today among adult competitive swimmers than it is among competitive athletes who specialize in other sports. Although chlorine is now known to be a lung irritant and swimming pool water is generally chlorinated, it would be rash to assume that frequent exposure to chlorine is the explanation of the high incidence of asthma among these swimmers, since           ________.


young people who have asthma are no more likely to become competitive athletes than are young people who do not have asthma.

competitive athletes who specialize in sports other than swimming are rarely exposed to chlorine.

competitive athletes as a group have a significantly lower incidence of asthma than do people who do not participate in competitive.

until a few years ago, physicians routinely recommended competitive swimming to children with asthma, in the belief that this form of exercise could alleviate asthma symptoms.

many people have asthma without knowing they have it and thus are not diagnosed with the condition until they begin engaging in very strenuous activities, such as competitive athletics.

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情景：游泳运动员比其它运动员更容易患上哮喘。虽然chlorine这个东西是一个诱发因素并且在游泳池里很多，但是不能擅自下一个“是chlorine导致患上哮喘”的结论。

推理：问题是让我们解释为什么不能说“是chlorine导致患上哮喘”。本题“正常”的逻辑为：
前提：接触chlorine的人比不接触chlorine的人更容易患哮喘。
结论：是chlorine导致患上哮喘
本题相当于问这个推理的削弱。
由于前提中chlorine和哮喘是相关的，结论中两者是导致关系，所以本题为相关因果推理。

选题方式：答案选项要么质疑因果的联系，要么给患上哮喘另外一个原因。

选项分析：

A选项：患上哮喘的人不会比没患上的人更有可能变成运动员。本选项没有提及是否会变成“游泳运动员”，因此和推理无关。（当然，就算是变成了游泳运动员，也是加强了推理）

B选项：除了游泳外的运动员很少会接触到chlorine。本选项更能加强“chlorine导致患上哮喘”这一结论。因为其它运动员是不患哮喘的。

C选项：运动员比非运动员患上哮喘的几率低。本选项和推理文段无关，即，我们要讨论的是chlorine和哮喘的关系，不是运动员和非运动员与哮喘的关系。

D选项：Correct. 直到几年前，医生会建议有哮喘的小孩练游泳，认为这种形式的运动会降低哮喘的症状。本选项质疑了因果间的联系，即，不是因为接触了chlorine才哮喘，而是因为哮喘的小孩都去练游泳了，这些游泳运动员在小时候本身就是哮喘患者，和chlorine无关。

E选项：直到参加剧烈的体育运动之前，很多人并不知道自己患有哮喘，并且没有去治疗。本选项讨论的是患有哮喘的人是否自己知情的问题，和为什么会患病无关。

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