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  3. The problem of acid rain originated with the Industrial Revolution, and it has been growing ever since. The more accurate scie

The problem of acid rain originated with the Industrial Revolution, and it has been growing ever since. The more accurate scie

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问题     The problem of acid rain originated with the Industrial
Revolution, and it has been growing ever since. The more
accurate scientific term of " acid rain" would be " acid
deposition". The dry form of such deposition is just damaging【M1】______
to the environment as the liquid form. The severity of its effects
has long been recognized in local settings, exemplified by the【M2】______
spells of acid smog in heavy industrialized areas. The widespread【M3】______
destructiveness of acid rain, moreover, has become evident only【M4】______
in recent decades. One large area has been studied extensively【M5】______
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 has been damaged by acid【M6】______
rain. The northeastern United States and eastern Canada have
also been particularly effected by this form of pollution; damage【M7】______
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 atmosphere are complex and as yet【M8】______
little understood, industries have tended to challenge such
assessments and to stress the need of further studies; and【M9】______
because of the cost of pollution reduction, governments have
tended to support this attitude.
    Studies released by the US government in the early 1980,【M10】______
however, strongly indicated industries as the main source of
acid rain, in the eastern US and Canada.
【M6】
选项
答案has→had
解析 语法错误。此句中主句的谓语indicated是过去式，根据时态一致原则，从句中应当使用过去完成时或一般过去时，又因为从句的动作发生在主句动作之前，所以此处应为过去完成时。
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