单项选择题

Researchers generally agree that acid rain arises primarily from the burning of coal and oil. The emissions contain oxides of nitrogen and sulfur, which can be carried hundreds of miles on the prevailing westernly winds. They eventually return to earth as dry gases and particulates and react with ground waters to form nitride or sulfuric acid. They may also be converted to the acids in the atmosphere by chemical processes that are still not fully understood. However the acids are formed, they have already been charged with a substantial list of damages. Corrosion of metal structures in windward areas of the country has been well-documented, but is a relatively minor entry on the list. Other forms of damage are far more insidious and longer-lasting. There is evidence, for example, that the acids leach heavy metals--aluminum, lead, and mercury-from the soil, which then poison plant roots and deprive them of vital nutrients. In lakes and rivers the acid alters the blood chemistry in fish. The metals, meanwhile, irreversibly damage the gill systems of certain species, such as bass and walleye. As the concentration of poison rises, egg hatching is reduced and the number of deformed embryos increases. Effects of acid rain are more severe in the eastern part of the country, for two reasons. First, most coal burning occurs in Ohio, Illinois, and Indiana, according to Eville Gorham, professor of ecology and botany at the University of Minnesota and a modern pioneer in the study of acid rain. These states pro- duce about a quarter of all U. S. sulfur oxide emissions, he told this year’’s American Chemical society meeting in Las Vegas, and the airborne particulates tend to wind up in the Northeast. The second reason is the varying chemical makeup of North American soils. Western soils are relatively rich in neutralizing compounds like magnesium and calcium carbonates, but the coarse, shallow eastern soils have much less of this buffering capacity. Together, these factors explain what Gorham calls a "pool of acid air pollution" over the Northeast that is strongly suspected of having killed or injured aquatic life in more than 200 lakes in New York’’s Adirondack region. According to the passages acid rain _________.