What is the main work of Himalayan rivers in their upper course?

Correct answer: Erosion

The direct answer is A, erosion. In the upper course, a Himalayan river flows down a steep mountain slope. Because the height difference and speed are comparatively great, the river cuts its bed and valley, carrying away rock, soil, and sediment. Reasoning chain: upper course means mountains; mountains usually have a steep gradient; a steep gradient gives the river strong downward and forward energy; strong energy produces vertical and lateral erosion. Option A is correct because erosion is the dominant work in the upper course, although transport can also occur. Option B is wrong because deposition is generally more important where the river becomes slower, especially in flatter lower areas; “only” also makes it too absolute. Option C is wrong because sea tides are caused mainly by the gravitational pull of the Moon and Sun, not by river work. Option D is wrong because plains may be developed later through deposition, but the upper course is not defined by only making plains. Exam cue: upper course means steep slope and erosion; lower course means slower flow and more deposition.