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


Read this paragraph and choose the correct option from the provided answers:

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.

 Choices:

- a lake

- a river

- a riverbank

- a waterfall


A:
a waterfall