2. 外语
  3. Acid ran leads to fish mortality. Many species of fish cannot survive in aquatic environments there the pH is below 5.0. If the

Acid ran leads to fish mortality. Many species of fish cannot survive in aquatic environments there the pH is below 5.0. If the

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问题      Acid ran leads to fish mortality. Many species of fish cannot survive in aquatic environments there the pH is below 5.0. If the water is too acid, the gill .systems of many kinds of fish can be damaged. 【C1】______ , the acid alters the blood chemistry of all fish. As a result, the fish population in an acidic environment decreases 【C2】______ becomes extinct. Approximately 20 percent of the lakes in Scandinavia are without fish. Moreover, in Nova Scotia the 【C3】______ salmon industry may be threatened by the decrease of salmon in rivers and streams. The impact is also 【C4】______ in the waters of Ontario and Quebec, where fish populations are 【C5】______ the decline.
     What potential remedies exist for the acid rain 【C6】______? The experts disagree. Some say new environmental laws should be 【C7】______ to control the emission of pollutants in the atmosphere. Some say that if we had known how serious acid rain was, we would have planned 【C8】______ to prevent it. 【C9】______, all agree that if the consumption of fossil fuel were reduced, we would have less of a problem. Another 【C10】______ is that special scrubbers could be installed in smokestacks to remove a good 【C11】______ of the pollutants before they get into the atmosphere. Other ideas even include breeding more 【C12】______ fish. And research suggests that spreading lime into lakes may be effective in 【C13】______ acidity. In conclusion, it is clear that if we truly want to reduce the impact of acid rain, a 【C14】______ of remedies and international cooperation must be explored. It is a sad fact that acid rain probably could have been avoided if we 【C15】______ what we know now.
【C15】______
选项 A、know
B、knew
C、had known
D、have known
答案C
解析 对过去事实的虚拟，主句是could have been avoided，从句应该用过去完成时(had done)，所以C是正确答案。
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