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.
为什么我感觉没有一个选项是对的呢?答案是C,请各位NN指点~~~
还是从正确理解题意入手：题目说B地在20年前颁布了法规，要求开采浅层煤矿的公司为挖土回填买单。20年来，回填技术并没有进步（联系后面，可以排除是由于科技进步原因导致回填成本下降）。但是，现在（20年后）的平均（注意这个字眼）回填成本只有XXX（具体数据可以忽略，重要的是后面给出的与之前成本的高低比较），比当初新法规刚刚颁布后的回填成本要少了一半还多。然后问怎么解释这一现象。
C是说那些挖煤的公司现在已经（说明从前不是，这里存在了比较，从而为差异打下伏笔）基本上停止了开采山区的浅层煤矿，因为在这些地方开采的话回填成本特别高。所以答案揭晓了，因为挖煤公司现在的策略是：放弃回填成本高的煤矿，而选择回填成本低的煤矿，所以现在的回填成本较之以前明显下降了。
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