Answer the following question given this paragraph: 

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


Q: When a stream flows from a hard to  soft rock area, what is often formed?

 Choices:

- a lake

- a river

- a riverbank

- a waterfall

A:
a waterfall
