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

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

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问题    Acid rain leads to fish mortality. Many species of fish cannot survive in aquatic environments where the pH is below 5.0. If the water is too acid, the gill systems of many kinds of fish can be damaged.【21】, the acid alters the blood chemistry of all fish. As a result, the fish population in an acidic environment decreases【22】becomes extinct.
Approximately 20 percent of the lakes in Scandinavia are without fish. Moreover, in Nova Scotia the【23】salmon industry may be threatened by the decrease of salmon in rivers and streams. The impact is also【24】in the waters of Ontario and Quebec, where fish populations are【25】the decline.
   What potential remedies exist for the acid rain【26】? The experts disagree. Some say new environmental laws should be【27】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【28】to prevent it.【29】, all agree that if the consumption of fossil fuel were reduced, we would have less of a problem. Another【30】is that special scrubbers could be installed in smokestacks to remove a good【31】of the pollutants before they get into the atmosphere. Other ideas even include breeding more【32】fish. And research suggests that spreading lime into lakes may be effective in【33】acidity. In conclusion, it is clear that if we truly want to reduce the impact of acid rain, a【34】of remedies and international cooperation must be explored. It is a sad fact that acid rain probably could have been avoided if we【35】what we know now.  
选项 A、know
B、knew
C、had known
D、have known
答案C
解析
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