Explain how economic activities have led to air, water, and land pollution

 Air pollution problems such as smog and acid rain result from the presence of, and interactions among, various air pollutants released to the atmosphere through natural processes and human activities. Natural sources of air pollution include forest fires, volcanoes and emissions of volatile organic compounds  from vegetation. Human sources of air pollution include activities that rely on carbon-based fuels, for example, transportation, electric utilities, industrial processes such as oil and gas production, as well as certain products, such as paints and solvents. Deforestation carried out to create dry lands is one of the major concerns. Land that is once converted into a dry or barren land, can never be made fertile again, whatever the magnitude of  measures to redeem it are. Land conversion, meaning the alteration or modification of the original properties of the land to make it use-worthy for a specific purpose is another major cause. This hampers the land immensely. Also there is a constant waste of land. Unused available land over the years turns barren; this land then cannot be used.Agriculture disrupts all freshwater systems hugely from their pristine states 

Why you put last one a? i think is b.