Q: You are presented with the quiz "What causes air pollution?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "burning wood is used to produce heat" and "air pollution is created by burning". So, what's the best answer to the question?

Choose from: 1. Smoking; 2. cigarettes; 3. producing heat; 4. Electrical energy; 5. oxygen; 6. chemical reaction; 7. wind turbines; 8. clouds;
The answer is:
3.