Final answer:
The primary source of air pollution in Beijing is the burning of coal, followed by emissions from motor vehicles and industrial activities.
Step-by-step explanation:
The main source of air pollution in Beijing is the combustion of fossil fuels. The burning of coal is particularly significant due to its high levels of pollutants and its widespread use in power stations and industries within the region. This activity, along with emissions from motor vehicles and other industrial processes, contributes to the poor air quality that plagues the city. The government and citizens are actively seeking ways to reduce this pollution, with some people measuring pollution levels independently to promote awareness and change. While riding bikes, burning natural gas, and other activities do have an impact, they are not the primary source of air pollution in Beijing.