What causes rocks in a stream to become smooth?

Answer: B. Water currents

The correct answer is B) Water currents.
Rocks in a stream become smooth primarily through a physical process called abrasion. As water currents move, they carry the rocks downstream, causing them to tumble and collide with one another and the stream bed. This constant friction and "bashing" acts like sandpaper, gradually wearing down the sharp, jagged edges of the rocks until they become rounded and smooth

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