Answer:
Walking
Explanation:
Global warming it's because greenhouse gases can't escape the atmosphere.
The most common greenhouse gas is CO2.
All kind od combustion generates CO2.
Driving cars is the same as combustion fuel unless the car is electric.
And releasing gases will also contribute to the greenhouse effect.
The thing goes more and more difficult every year because trees are the ones that "cleans" the gases, but deforestation won't stop.
Walking is good for your health and will not produce any residues to the environment.
