Passage Summary: Scientists used to think this long-necked dinosaur ate leaves from high trees, but new models show its neck couldn't actually bend upward. Since it could bend down, they conclude it must have eaten low-lying or underwater plants.

Conclusion: The Diplodocus must have consumed vegetation found on the ground or in the water.

Reasoning: Computer models indicate that the dinosaur's neck structure was physically incapable of reaching high-growing leaves, though it could easily reach downward.

Analysis: The argument relies on a 'gap' between the dinosaur's neck mobility and its actual feeding habits. It assumes that if the neck couldn't reach up, the dinosaur couldn't reach high plants by any other means, such as rearing up on its hind legs. To find the necessary assumption, ask yourself what *must* be true for the conclusion to hold water. If the dinosaur could reach high leaves without bending its neck upward, the whole argument falls apart, so the author must assume no such alternative existed.