Here's a problem to solve: Soils change both through natural processes and as a result of human activity. Which of the following soil changes is due only to natural causes?

Among the 4 following options, which is the correct answer?

- A: degradation of nutrients due to pesticides
 
- B: formation of deserts due to tree felling
 
- C: flooding due to dam construction
 
- D: removal of nutrients due to heavy rains
 