Answer:
The correct answer is D. Encourage citizens to walk or use bikes to travel
Step-by-step explanation:
The main sources of gas emissions include manufacturing, vehicles, fires, among others because all of these activities release gases such as carbon dioxide that are nocive for the environment and lead to air contamination.
In this context, one-way emission levels can be reduced is by encouraging citizens to walk or ride bikes rather than using cars. This is because cars release gases that increase the emission levels but walking or riding a bike does not imply the release of nocive gases or substances.