Answer:
Carbon dioxide.
Step-by-step explanation:
The carbon dioxide constitute the majority of greenhouse gas emission. This is produced by large industries, oil exploitation to get energy, massive felling of trees, combustions of fossil fuels like oil and natural gas.
The oil industry is one of the most contaminant in the world, because emits a huge amount of carbon dioxide. Therefore, they way to reduce this intense contamination is using nuclear energy, because it doesn't contaminates at all, even nuclear sources can produce more powerful energy.
In addition to that, trees makes up the principal oxygen source, so that planting them will help to process more carbon dioxide from the atmosphere, converting it in oxygen or clean air.