Given a scientific passage and an answer, generate a question for the given answer.
One example is below.
Q: Passage: A frameshift mutation is a deletion or insertion of one or more nucleotides that changes the reading frame of the base sequence. Deletions remove nucleotides, and insertions add nucleotides. Consider the following sequence of bases in RNA:. Answer: nucleotides.
A: A frameshift mutation is a deletion or insertion of one or more of what that changes the reading frame of the base sequence?
Rationale: The science passage defines frame mutation and the main compoment here is nucleotides. Hence the generated question is apt.
Q: Passage: 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. Answer: a waterfall
A:
When a stream flows from a hard to soft rock area, what is often formed?