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. Ttherefore, if such direct transmission occurs, the disease cannot be eradicated by __. Which one of the following best completes the argument? ['destroying all cattle in areas where BSE occurs and raising cattle only in areas to which BSE is known not to have spread', 'developing a drug that kills the agent that causes BSE, and then treating with that drug all cattle that might have the disease', 'removing from the herd and destroying any diseased animal as soon as it shows the typical symptoms of advanced BSE', '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']