Is lichen growing on rocks a mechanical or a chemical

Asquirrel population lives in an area. over many years, a river in that area grows wider and stronger, eventually forming a canyon. the squirrel populations on the two sides of the canyon can no longer mate with each other and ultimately form distinct species. this is an example of

When ash and dust begin to settle on the ground, they eventually become compacted by

True or false multiple lane changes must be done individually going through the same procedures for each lane change