Concept:Potholes are circular hollows carved into a riverbed by the grinding action of pebbles carried by the water.
Explanation:In the upper course, the river flows fast and its load includes many pebbles and stones.
Some of these pebbles settle into small cracks or hollows on the rocky riverbed.
The force of moving water makes the pebbles spin and rotate rapidly inside these hollows.
This continuous grinding and drilling action wears away the bed rock, making the hollows deeper and wider.
After a long period of time, these enlarged hollows form circular depressions known as potholes.
Potholes are therefore typical erosion features of a river’s upper course, created by corrosion and abrasion.
Answer:A. Potholes