我国的黄土高原，根据历史记载和埋藏的土壤证明，这里过去不仅有肥沃的草原，而且有茂密的森林，大量的野生动物在那里繁衍、栖息。然而经过几百年的掠夺式开发，滥伐森林，超载放牧，破坏了生态平衡，因而造成了现在这一片荒山秃岭，水土严重流失的情况，要恢复昔日的面貌已经十分困难了，据此事实分析回答以下问题。
以上事实说明，造成世界范围的许多生态系统被破坏的人为因素之一就是______。
A．对自然资源的不合理利用
B．滥伐森林
C．开采石油和矿山
D．超载放牧
参考答案：A
解析： 本题是一道环境保护知识题，考查环保知识。滥伐森林、超载放牧、开采石油和矿山都是破坏生态系统的具体行为，因此应填为对自然资源的不合理利用。
I ran across an old photo of him the other day, thinking of some old things. He’s been dead for 25 years. His name was Rex.
36 was his favorite recreation(娱乐). He had so much 37 in the water as any person I have known. You didn’t have to throw a stick in the water to 38 him to go in. Of course, he would bring back a stick to you if you 39 throw one in. That 40 me of that night, 41 he brought back a small box that he found somewhere--- how 42 nobody ever knew. Since it was Rex, it 43 easily have been half a race. The box wasn’t a good one. It was just a 44 old piece that somebody 45. Still it was something he wanted, probably 46 there was some difficulty in transportation（运输）. And that he thought could test his courage. We first knew about his achievement when, deep in the night, we 47 him trying to get the box up onto the porch(门厅). It sounded 48 two or three people were trying to tear the house 49 . We came downstairs and turned on the 50 light. Rex was on the top step trying to pull the thing up, but it had 51somehow. And he was just holding his own(坚持着). I suppose he would have held his own 52 dawn if we hadn’t helped him. The next day we carried the box miles away and threw it out. If we had thrown it out in a 53 place, he would have brought it home again, as a small token(象征)of his strength in such matters. 54, he had been taught to carry heavy wooden objects about and he was 55 of his skill.
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