T-4-Q2（答案是C，不明白为什么啊，请NN指教，非常感谢~）
Twenty years ago, Balzania put in place regulations requiring operators of surface mines to pay for the reclamation of mined-out land. Since then, reclamation technology has not improved. Yet, the average reclamation cost for a surface coal mine being reclaimed today is only four dollars per ton of coal that the mine produced, less than half what it cost to reclaim surface mines in the years immediately after the regulations took effect.
Which of the following, if true, most helps to account for the drop in reclamation costs described?
A. Even after Balzania began requiring surface mine operators to pay reclamation costs, coal mines in Balzania continued to be less expensive to operate than coal mines in almost any other country.
B. In the twenty years since the regulations took effect, the use of coal as a fuel has declined from the level it was at in the previous twenty years.
C. Mine operators have generally ceased surface mining in the mountainous areas of Balzania because reclamation costs per ton of coal produced are particularly high for mines in such areas.
D. Even after Balzania began requiring surface mine operators to pay reclamation costs, surface mines continued to produce coal at a lower total cost than underground mines.
E. As compared to twenty years ago, a greater percentage of the coal mined in Balzania today comes from surface mines.
因该是这样的吧~C中. Mine operators have generally ceased surface mining in the mountainous areas of Balzania because reclamation costs per ton of coal produced are particularly high for mines in such areas.，由于山区表层采矿成本高就停止了（ceased）,也就是说，剩下的都是非山区，成本自然低下来吧~呵呵，你觉得这样解释可好？
文中说“Yet, the average reclamation cost for a surface coal mine being
reclaimed today is only four dollars per ton of coal that the mine produced, less
than half what it cost to reclaim surface mines in the years immediately after the
regulations took effect.”
请注意 the average reclamation cost，平均费用的算法即使把所有surface mines的费用加
起来，然后除以mines的个数。
C. Mine operators have generally ceased surface mining in the mountainous areas
of Balzania because reclamation costs per ton of coal produced are particularly
high for mines in such areas.
刚开始的时候所有的矿都在开采，所以平均费用会相对偏高，后来当老板意识到mountainous
areas of Balzania 的开采费用很高之后，就将它们关闭了，之开采比较便宜的矿。进而，整
个开采费用平均水平就拉低了，正好解释了这一现象。
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