Water that flows downhill along earth's surface is called

Water that flows downhill along the earth’s surface is called runoff. Runoff or also called as surface water runoff or overland flows is the flow of water when there’s excess water caused by ice melted, rain or storm. These occur because maybe the rain or the storm pours so hard and quick that the soil can’t absorb it fastly. If runoff water flows on the ground and reached a channel, it is called as nonpoint source. One example of nonpoint source are the canals full of leaves, the water can’t continue flowing because the leaves stops it.

Answer:   [a]:   "slate" .

  concave means having an outline or surface that curves inward like the interior of a circle or sphere. so the mirror curves inward.