leaching of soils evaporation rates runoff from rains amounts of sunlight
I gave my students this multiple choice question: Scientists have studied the productivity of crops in mountain valleys. In some areas, the valleys are more productive than others. The increase in which factor most likely accounts for the high productivity of some areas in mountain valleys?

Only one answer is correct among these 4 choices:
- leaching of soils
- evaporation rates
- runoff from rains
- amounts of sunlight

Could you tell me which one is correct?

runoff from rains