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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 identifi ed 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_____.

Achickens 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
BFerber'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
Cresistance to some infectious organisms such as the virus that causes Ferber's syndrome can be acquired by exposure to a closely related infectious organism
Dchickens 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 fi rst year of life
Ethe 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

正确答案: D

求牛人一撇

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chasingM7

楼主

发表于 2011-8-2 21:50:40 | 显示全部楼层

conclusion: This fact, however, does not indicate that most chickens are immune to the virus that causes Ferber’s syndrome

Premises: 1) 家畜通过吃的东西感染综合症。
2）鸡也吃这种饲料，但是很少发现有这种综合症。
3）即便鸡很少发现，也不能说鸡对这种综合症有免疫能力。

weaken，就是要找一个原因，导致了即使鸡感染了，也无法发现。

D）说明了这个条件。如果这个综合症的发病时间较长（一年），而鸡一般很快就被就会被送上餐桌（一年之内）。它因削弱。

hope it helps!