A、Because they burn very fast. B、Because they have water inside. C、Because people tend to burn them at the same time. D、Because

Burning fallen leaves used to be standard practice across North America, but most districts now ban or discourage this practice due to the air pollution it causes.
    What’s more, because of the moisture that is usually trapped within leaves, they tend to burn slowly and thus generate large amounts of particles in the air, such as bits of dust, smoke and other solid materials. These particles can reach deep into lung tissue and cause coughing, chest pain, shortness of breath and other breathing problems.
    Individual leaf fires from time to time usually don’t cause any major pollution, but multiple fires in one geographic area can cause concentrations of air pollutants that exceed federal air quality standards. According to the U.S. Environmental Protection Agency(EPA), several leaf and yard waste fires burning simultaneously in a particular place can cause air pollution which equals that from factories, motor vehicles and lawn equipment.
    On the contrary, leaves can be made fertilizer, which is the most eco-friendly alternative to burning. Dry leaves alone will take a long time to break down, but mixing in green plant materials, such as grass, will speed up the process. Sources of nitrogen, such as livestock wastes or commercial fertilizer, will also help.
    Another option is to chop leaves for use as cover for your lawn or to help protect garden and landscape plants. But before you add no more than a two-to-three-inch layer of leaves around actively growing plants, chop the leaves first so they don’t prevent air from reaching roots.
19. Why do most cities discourage burning fallen leaves?
20. Why can fallen leaves cause air pollutants?
21. What is the most eco-friendly way to deal with fallen leaves?