单项选择题

The word "smog" has become a household word in urban China. Smog is an 1 of greenhouse gases and pollutants that reduce visibility and harm respiratory functions. Smog is typically 2 cities with high concentrations of cars and factories. The population density, amount of industry and the fuels used 3 together to have an impact on smog levels. During summer, smog is worse 4 the production of ozone, the main component of smog, increases in strong sunlight. The important thing to understand about smog is that this kind of pollution is spread out 5 large distances.
Walking, biking or using public transportation can help limit ozone production. 6 , decreasing household electricity use and keeping your vehicles fuel-efficient reduces 7 greenhouse gases. Checking tire pressure, oil levels, air filters, and getting regular maintenance help 8 fuel efficiency. Be sure to use only the fuel recommended in the vehicle’s user 9 . Simple steps like avoiding stop-and-go traffic and reducing vehicle workload decrease smog-related emissions. To lighten the workload, avoid running the air-conditioner, 10 the engine and carrying heavy objects in the vehicle.