Here's a problem to solve: Certain lakes in the northeastern part of the United States have had decreases in pH levels, causing the deaths of many fish. What is the most likely reason for this change in pH levels?

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

- A: nuclear power plants
 
- B: hydroelectric power plants
 
- C: coal-burning factories
 
- D: noncommercial farms
 