Air pollution involves the release into the atmosphere【C1】______gases, finely divided soli

Air pollution involves the release into the atmosphere【C1】______ gases, finely divided solids, or finely dispersed liquid aerosols at rates that【C2】______ the capacity of the atmosphere to dissipate them or to dispose【C3】______ them through incorporation into solid or【C4】______ layers of the biosphere. Air pollution【C5】______ a variety of causes, not all of which are【C6】______ human control. Dust storms in desert areas and smoke from forest and grass fires【C7】______ to chemical and particulate pollution of the air. Forest fires that swept the state of Victoria, in Australia, in 1939 caused【C8】______ air pollution in Queensland, more than 2,000 miles (3,000 kilometres) away. Dust blown from the Sahara has been【C9】______ in West Indian islands. The discovery of 【C10】______ in Antarctica, where they have never been used, suggests the【C11】______ to which aerial transport can carry pollutants from one place to another. Probably the most important natural【C12】______ of air pollution is volcanic activity , which at times【C13】______ great amounts of ash and toxic【C14】______ into the atmosphere. The eruptions of such volcanoes as Krakatoa, in the East Indies, Mt. St. Helens, in Washington, and Katmai, in Alaska, have been 【C15】______ to 【C16】______ climatic changes.

Air pollution may affect humans directly, causing a smarting of the【C17】______ or coughing. More indirectly, the effects of air pollution are experienced at【C18】______ distances from the source, as, for example, the fallout of tetraethy lead from urban automobile【C19】______ which has been observed in the oceans and on the Greenland ice sheet. Still【C20】______ directly experienced are the possible effects of air pollution on global climates.

【C1】

A．at

B．of

C．by

D．with