Answer the following question given this paragraph: 

Mountain streams may erode waterfalls. A waterfall forms where a stream flows from an area of harder to softer rock ( Figure below ). The water erodes the softer rock faster than the harder rock. This causes the stream bed to drop down, like a step. This creates a waterfall. As erosion continues, the waterfall gradually moves upstream.


Q: What process is involved in the formation of a waterfall, when a stream flows from an area of harder to softer rock?

 Choices:

- calcification

- migration

- evaporation

- erosion

A:
erosion
