It is not known whether bovine spongiform encephalopathy (BSE), a disease of cattle invariably deadly to them, can be transmitted directly from one infected animal to another at all stages of the infection. If it can be, there is now a reservoir of infected cattle incubating the disease. There are no diagnostic tests to identify infected animals before the animals show overt symptoms. Therefore, if such direct transmission occurs, the disease cannot be eradicated by __________.

Which one of the following best completes the argument?

[A]. removing from the herd and destroying any diseased animal as soon as it shows the typical symptoms of advanced BSE
[B]. developing a drug that kills the agent that cause BSE, and then treating with that drug all cattle that might have the disease
[C]. destroying all cattle in areas where BSE occurs and raising cattle only in areas to which BSE is known not to have spread
[D]. developing a vaccine that confers lifelong immunity against BSE and giving it to all cattle, destroying in due course all those animals for which the vaccine protection came too late
[E]. developing a diagnostic test that does identify any infected animal and destroying all animals found to be infected
Answer: A

Read the fragment of the final sentence carefully: The correct choice must describe a program that won’t work. Well, if BSE can be transmitted from cow to cow at any and all stages of the disease, then diseased cows without visible symptoms could be passing BSE around unnoticed. And there’d be no way to identify those cows, since no diagnostic test can identify cows with BSE but without symptoms. Our search for the non-solution doesn’t last long, because (A) is it. Removing and destroying cows that actually showed symptoms wouldn’t control the disease if it can be spread at any stage: Sick cows would already have been passing along the disease before they showed symptoms. So (A)’s plan would not fell the disease.

(B) When you gave (B)’s drug to a cow, it would have no disease-bearing agent to spread to other cows, and the disease couldn’t spread. So if you treated all cattle that might have the disease, you could eradicate it.

(C) would work like a charm: All the cows that could possibly be carrying the disease would be eliminated. Sad for the cows, but good for eliminating BSE.

(D), which would immunize the healthy animals and kill off the diseased ones, would have a solid shot at eradicating BSE.

(E) A diagnostic test that could identify the diseased animals right from the start would solve the problem: All diseased animals could be eliminated before they transmitted the disease. Result? No more BSE.

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