In the lower part of the Yellow River, the river bed is about 10 meters higher than both sides of the river, so that it is also called "overground river". This is due to the large sediment content of the Yellow River, slow water speed and continuous sedimentation in the downstream. What caused the "ground river"? ['The lower reaches of the Yellow River were flood-protected, causing the dam to be built too high.', 'The sediment content of the Yellow River is large, and the sediment continues to deposit due to the slow water speed in the downstream.', 'The Yellow River often stops.', 'The terrain passed by the lower Yellow River was higher.']