How did a smooth rock become smooth?

A smooth rock typically becomes smooth through a process of abrasion and weathering, which often occurs when it is subjected to running water over an extended period. In a stream or river, the constant movement of water causes the rock to collide with other particles, such as dirt, sand, and other rocks. This repeated friction wears away rough edges and contributes to the smoothing of the rock's surface.

The other choices provide different processes that might influence the rock's characteristics, but they do not directly explain the reason for the smoothness as effectively as the long-term exposure to a stream. Polishing involves artificial intervention, while being submerged does not ensure the same level of abrasion as flowing water. Wind erosion can affect rocks, but it primarily operates differently and often does not produce the same kind of smoothness associated with water erosion. Hence, the long duration in a stream is the key factor in the formation of a smooth rock.