constantly flowing water many fish in the river new soil deposited by the river warm water from an ocean
I gave my students this multiple choice question: What causes the soil in a flood plain to be generally rich in nutrients?

Only one answer is correct among these 4 choices:
- constantly flowing water
- many fish in the river
- new soil deposited by the river
- warm water from an ocean

Could you tell me which one is correct?

new soil deposited by the river