同许多北欧国家一样，美国的危险系数低。
The U.S. was ranked as low risk, along with many other countries in Northern Europe.
结果表明，降雪的酸雪频率各年均有不同，但都比同季中降雨的酸雨频率低；
The results show that the frequence of acid snow is different every year, but.
结果表明，降雪的酸雪频率各年均有不同，但都比同季中降雨的酸雨频率低；
The results show that the frequence of acid snow is different every year, but.
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