There can be no stream erosion or glacial erosion wothout

B) There can be no stream erosion or glacial erosion without gravity causing the water and glaciers to move down the mountains.

Explanation:

Streams and rivers cause erosion through hydraulic action and cavitation by hitting the banks and bed of its channel compressing air into gaps and forcing them to weaken. In the same was a glacier uses its weight and force of its movement to erode mountains. With glaciers and steams, they both have the potential to carry sediment or different sizes. They can use this sediment to help cause erosion faster, similar to sandpaper as it becomes more coarse.