Here's a problem to solve: Pesticides are used by farmers to control insects. Sometimes, the pesticides run off into streams. This is an example of

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

- A: farmers working to protect the environment.
 
- B: natural substances being used to solve problems.
 
- C: farmers trying to decrease the amount of crops.
 
- D: the solution for one problem creating another problem.
 