Which of the following most logically completes the argument?

Ferber's syndrome, a viral disease that frequently affects cattle, is transmitted to these animals through infected feed. Even though chickens commercially raised for meat are often fed the type of feed identified as the source of infection in cattle, Ferber's syndrome is only rarely observed in chickens. This fact, however, does not indicate that most chickens are immune to the virus that causes Ferber's syndrome, since _____.


chickens and cattle are not the only kinds of farm animal that are typically fed the type of feed liable to be contaminated with the virus that causes Ferber's syndrome

Ferber's syndrome has been found in animals that have not been fed the type of feed liable to be contaminated with the virus that can cause the disease

resistance to some infectious organisms such as the virus that causes Ferber's syndrome can be acquired by exposure to a closely related infectious organism

chickens and cattle take more than a year to show symptoms of Ferber's syndrome, and chickens commercially raised for meat, unlike cattle, are generally brought to market during the first year of life

the type of feed liable to be infected with the virus that causes Ferber's syndrome generally constitutes a larger proportion of the diet of commercially raised chickens than of commercially raised cattle

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情景：常发生在牛身上的Ferber综合症是通过被感染食物传播的。虽然肉鸡也被喂这种被感染食物，但Ferber综合症在肉鸡中很少被观察到。但是，这并不能说明肉鸡对Ferber病毒有免疫。

推理：本题是一个“现象解释型考题”。按照正常逻辑来说，即，结论应该为“肉鸡对Ferber病毒有免疫”。由于前提和结论描述的是不同的事件，且结论为因，所以本题是果因推理。
顺序的因果逻辑：因为肉鸡对Ferber病毒有免疫，所以Ferber综合症在肉鸡中很少被观察到
（果）前提：Ferber综合症在肉鸡中很少被观察到
（因）结论：肉鸡对Ferber病毒有免疫

选题方式：果因推理有两个评估方向，简而言之，即，答案选项中要么给出产生“Ferber综合症在肉鸡中很少被观察到”的另外一个原因，要么讨论“肉鸡对Ferber病毒有免疫”与“Ferber综合症在肉鸡中很少被观察到”之间的关系。

选项分析：

A选项：被喂食含Ferber病毒的饲料动物不仅仅是肉鸡和牛。本选项讨论的是畜牧类动物的特点，没有提及原文的因果。

B选项：在没有被喂食含Ferber病毒饲料的动物中也发现了Ferber综合症。无论其它动物中是否有Ferber综合症，其均和推理文段中的因果无关。

C选项：对感染性病毒（如Ferber病毒）的抵抗性可以通过暴露在相关病毒中来获得。本选项没有提及推理文段中的因果。

D选项： Correct. Ferber综合症在鸡和牛身上表现出来需要一年多的时间，而鸡在一年内就被卖了。本选项可以解释为什么“Ferber综合症在肉鸡中很少被观察到”，因为还没有发病就直接被卖掉了。属于CQ1：说明结果问题。

E选项： 鸡饲料中被Ferber病毒感染的比例大于牛饲料。饲料中含有病毒不一样导致吃这些饲料的动物含有病毒。

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