Problem:OPTIONS:
- soil
- cracks
- thrust
- rain
- biome
- spring
- potash
- tube

You are presented with the quiz "What is formed by rivers flowing over rocks?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "soil is formed by rocks eroding" and "rivers erode the rocks they flow over". So, what's the best answer to the question?
****
A: soil

Problem:OPTIONS:
- matter vibrating
- Pollution
- rain
- Stale air
- condensation
- emissions
- suffocation
- friction

You are presented with the quiz "What can gas furnaces cause?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "carbon monoxide is a gas produced by furnaces and other devices that burn fuel" and "excessive accumulation build-up of carbon monoxide can cause suffocation". So, what's the best answer to the question?
****
A: suffocation

Problem:OPTIONS:
- killing insects
- Make money
- survive
- Start fires
- Harm hosts
- Harm themselves
- protect them
- reproduce

You are presented with the quiz "What can roundworms do?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "roundworms may be free-living or parasitic" and "internal parasites harm their hosts in many ways". So, what's the best answer to the question?
****
A: Harm hosts

Problem:OPTIONS:
- watershred
- caves
- desert
- coral reef
- savannah
- streams
- loose soil
- nesting

You are presented with the quiz "What type of habitat can be formed by sediment moved downstream by water?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "asandbar is formed by water moving sediment downstream" and "sandbars are preferred nesting habitat". So, what's the best answer to the question?
****
A:
nesting