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?

  Options: 
 A. farming
 B. restoring wetlands
 C. species reintroduction
 D. excessive groundwater use
Answer: A