Naturalists have observed that a group of birds usually has a strict hierarchy. High-ranking birds oppress low-ranking birds. The darker the feathers on the head and the thicker the stripes of the breast feathers, the higher the ranking, and vice versa. Naturalists have also observed that the older the birds, the darker the feathers on their heads, and the thicker the stripes on the breast feathers. This shows that the status of birds in a group is gradually established through long-term common life. Which of the following, if true, would most effectively weaken the above argument? ['One cannot identify the position of white swans in the group by the color of the feathers on the head or the stripes of the breast feathers, because the color of the feathers on their heads does not distinguish between light and dark, and the breast feathers have no stripes.', 'People paint the head and breast feathers of a young lower bird with the color and stripes of the higher bird and place it in another group of birds of the same kind. This bird is treated higher in the new group.', 'If there is a strict hierarchy in the bird world, their respective status in a group of birds will not be unchanged for the life.', 'If there is a strict hierarchy in the bird world, then in a flock of birds, they will also fight to improve their status.']