Here's a problem to solve: One of the principal causes of acid rain is

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

- A: waste acid from chemical factories being pumped into rivers
 
- B: acid from chemical laboratories evaporating into the air
 
- C: gases from burning coal and oil dissolving in water in the atmosphere
 
- D: gases from air conditioners and refrigerators escaping into the atmosphere
 