Fossilized teeth of an extinct species of herbivorous great ape have on them phytoliths, which are microscopic petrified remains of plants. Since only phytoliths from certain species of plants are found on the teeth, the apes' diet must have consisted only of those plants. The argument assumes which one of the following? ['Plants of every type eaten by the apes left phytoliths on their teeth.', 'Phytoliths have also been found on the fossilized teeth of apes of other extinct species.', "None of the plant species that left phytoliths on the apes' teeth has since become extinct.", 'Each of the teeth examined had phytoliths of the same plant species on it as all the other teeth.']