farming restoring wetlands species reintroduction excessive groundwater use
I gave my students this multiple choice question: When European settlers first came to North America, the average thickness of the top layer of soil was approximately 23 cm. Today, the average thickness of the top layer of soil is less than 15 cm. Which of the following factors has most likely had the greatest effect on erosion of the top layer of soil?

Only one answer is correct among these 4 choices:
- farming
- restoring wetlands
- species reintroduction
- excessive groundwater use

Could you tell me which one is correct?

farming