The caterpillar of the monarch butterfly feeds on milkweed plants, whose toxins make the adult monarch poisonous to many predators. The viceroy butterfly, whose caterpillars do not feed on milkweed plants, is very similar in appearance to the monarch. Ttherefore, it can be concluded that the viceroy is so seldom preyed on because of its visual resemblance to the monarch. Which one of the following, if it were discovered to be true, would most seriously undermine the argument? ['Some of the predators of the monarch butterfly also prey on viceroys.', 'The viceroy butterfly is toxic to most predators.', 'Some predators do not have a toxic reaction to insects that feed on milkweed plants.', 'Toxicity to predators is the principal means of protection for only a few butterfly species.']