Currently, warm-blooded pets are allowed to enter the country only if they have a certificate showing that they have been vaccinated against rabies. This policy is flawed because vaccinations sometimes fail to take effect. If the vaccination is successful, the animal has a high level of rabies antibodies in its blood. Ttherefore, a more effective means of preventing pets with rabies from entering the country would be to administer blood tests and refuse entry to any pets whose blood did not contain a high level of rabies antibodies. Which one of the following, if true, most substantially weakens the argument? ['Under existing policies, some species of animals are quarantined and observed for symptoms of rabies before a decision is made to allow them into the country.', 'The proposed policy would be ineffective in preventing wild animals infected with rabies from entering the country.', 'A significantly larger percentage of rabies vaccinations than of vaccinations against other animal diseases fail to produce immunity.', 'There is no way to distinguish between high antibody levels caused by active infection and high levels caused by successful immunization.']