Paleontologists had long supposed that the dinosaur Diplodocus browsed for high-growing vegetation such as treetop leaves by raising its very long neck. But now computer models have shown that the structure of Diplodocus"s neck bones would have prevented such movement. The neck could, however, bend downward and even extend below ground level, allowing Diplodocus to access underwater vegetation from dry land. Thus, Diplodocus must have fed on plants on or near the ground, or underwater. Which one of the following is an assumption required by the argument? ['Diplodocus was not able to browse for underwater vegetation by kneeling beside bodies of water or by walking into them.', 'The same type of neck structure is found in modern ground-feeding animals.', 'Diplodocus was not able to see in front of itself unless its head was angled steeply downward.', 'Diplodocus had no other way of accessing high-growing vegetation, such as by rising up on its hind legs.']