酸雨对生态系统的破坏很大，被称为“空中死神”．酸雨形成的主要原因是（ ）
A．火山爆发
B．农药污染
C．大气中二氧化硫过多
D．二氧化碳污染
酸雨产生的原因主要是因为空气中的二氧化硫、二氧化氮等酸性氧化物过多，这些酸性氧化物结合水形成相应的酸性物质会引起酸雨．火山爆发能引起酸雨，但不是主要原因；农药污染会污染土壤和水，不会引起酸雨；二氧化碳污染会引起温室效应，不会形成酸雨．
故选：C．
| 完形填空。 A group of frogs were traveling 1 the forests, but suddenly two of them fell into a hole. The other frogs tried to help them. When they saw how deep the hole was, they cried to the two frogs that they could not be saved. The two frogs didn't 2 and tried their best to jump up out of the hole. The other frogs 3 saying that they were sure to die. 4 , one of the two frogs, who heard what the other frogs were saying, gave up. Then he fell down and died. The 5 frog, however, continued to jump as 6 as he could, and at last made it out. When he 7 , the other frogs asked, "Didn't you hear us?" The frog, who had a poor 8 , said, "I thought you were encouraging me all the time." The story teaches us a 9 :There is power (力量) of life and death in the tongue. An encouraging 10 to those who are down can help them out while a discouraging word can kill them. | ||||
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