Answer:
the very long river
Explanation:
Larger rocks tend to settle out of the water flow sooner than smaller rocks do, so the rocks remaining at the end of a very long river would tend to be very small. A very short river may still be carrying somewhat larger rocks.
The very long river probably drops the smallest rock pieces near the ocean.
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Perhaps the implied assumption here is that a very short river will have a higher gradient than a very long river. Another assumption is that rocks are picked up nearer the headwaters, and the gradient decreases with distance.
If both rivers end at a waterfall into the ocean, they may very well carry the same sort of rock size distribution. If the short river traverses muddy terrain, and the longer river traverses rocky terrain, the answer may be reversed.
