单项选择题

Acid rain, which is a form of air pollution, currently becomes a subject of great debate because of widespread environmental damage for which it has been blamed. It forms when oxides of sulfur (硫) and nitrogen (氮) combine with atmospheric moisture to yield sulfuric and nitric acids, which may then be carried long distances from their source before they drop in the form of rain. The pollution may also take the form of snow or fog or fall down in dry forms. In fact, although the term "acid rain" has been in use for more than a century — it is derived from atmospheric studies that were made in the region of Manchester, England — the more accurate scientific term would be "acid deposition". The dry form of such deposition is just as damaging to the environment as the liquid form.
The problem of acid rain originated with the Industrial Revolution, and it has been growing ever since. The severity of its effects has long been recognized in local settings, as exemplified by the spells of acid smog in heavily industrialized areas. The widespread destructiveness of acid rain, however, has become evident only in recent decades. One large area that has been studied extensively is northern Europe, where acid rain has eroded structures, injured crops and forests, and threatened life in freshwater lakes. In 1984, for example, environmental reports indicated that almost half of the trees in Germany’s Black Forest had been damaged by acid rain. The northeastern United States and eastern Canada have also been particularly affected by this form of pollution. Damage has also been detected in other areas of these countries and other regions of the world.
Industrial emissions have been blamed as the major cause of acid rain. Because the chemical reactions involved in the production of acid rain in the atmosphere are complex and as yet little understood, industries have tended to challenge such assessments and to stress the need for further studies and because of the cost of pollution reduction, governments have tended to support this attitude. Studies released by the USA government in the early 1980s, however, strongly indicated industries as the main source of acid rain, in the eastern USA and Canada.
According to the passage, acid fog _____.