Scientists have shown that older bees, which usually forage outside the hive for food, tend to have larger brains than do younger bees, which usually do not forage but instead remain in the hive to tend to newly hatched bees. Since foraging requires greater cognitive ability than does tending to newly hatched bees, it appears that foraging leads to the increased brain size of older bees. Which one of the following, if true, most seriously weakens the argument above? ['The brains of older bees that stop foraging to take on other responsibilities do not become smaller after they stop foraging.', 'In some species of bees, the brains of older bees are only marginally larger than those of younger bees.', 'Those bees that travel a long distance to find food do not have significantly larger brains than do bees that locate food nearer the hive.', 'The brains of older bees that never learn to forage are the same size as those of their foraging counterparts of the same age.']