Why are rocks found at the bottom of a river smooth and flat?

The rocks found at the bottom of a river smooth and flat because those are collides with each other.

Explanation:

At the bottom of the river, the rocks which are found are very smooth and the shape of these rocks are flat because the rocks at the bottom collide with each other.

This results in the chipping off parts of the rocks and this results in the surface of the rock to become smooth and becoming flat. Moreover the sand found at the bottom acts as a sand paper creating resistance and smooth-ens the rocks at the bottom of the river.

its i think c because its the lowering cloud

i beleive d is the right answer