86.H argued that grazing land held in common would always be used less carefully than private grazing land .Each rancher would be tempted to overuse common land because the benefitswould accure to the individual, while the costs reduced land quality that results from overuse would be spread among all users. But a study comparing 217 million acres of common grazing land with 433 million acres of private grazing land showed that the common land was in better condition.

which  of the following, if true and known by the ranchers, would best help explain the results of the study?

A: with private grazing land, both the costs ad the benefits of overuse fall to individual user

B:the cost in reduced land quality that is attributable to any individual user is less easily measured with common land than it is with private land.

C:An individual who overuses common grazing land might be able to achieve high returns than other users can, with the result that he or she would obtain a competitive advantage

D:If one user of common land overuses it even slightly, the other users are likely to do so even more, with the consequence that the costs to each user outweight the benefits.

E.There are more acres of grazing land held privately than there are held in common.

答案选D，对于d的前半句话和consequence……的关系不太理解；为什么前面的行为会导致成本大于好处？

成本是什么？

这题的意思就是说：

大家都知道“一个人过渡使用公共牧场会导致大家一起过渡使用，牧场最终恶化”，所以大家都保护公共牧场，对吗？

前提：公用牧场比私人牧场较少得到照顾，因为使用公共的，个人得到利益，却集体进行投入

结论：公用牧场比私人牧场的状况还好

答案：如果一个人使用公共的，大家就都用公共的，平摊到每人头上的钱大于收入

解释方法：其他原因导致结论，同时不违背前提，即前提结论都成立

总感觉楼上的解释有些问题。个人认为：

1.调查的结果是与Hardin's claim矛盾的，必须找到造成矛盾的原因，既“为什么放牧者会公共牧场没有被过度使用或少用了”（前提是在开始使用前，公共牧场和私人牧场的质量是可比的，不会一个很坏，一个很好，本题前的85题已经讨论了这个前提）。

所以四个选项中只有D讲到了“少用”＝overuse it even slightly。虽然不影响正确结果，但是大家还是希望弄清楚选项D在讲什么东东。

2.根据原文，在利己主义的驱动下，放牧者认为在在公共牧场放牧，获益是自己的，导致牧场质量下降的成本是分摊给大家的（利己不利人）；那么如果放牧者认为在公共牧场放牧的成本高于获益（没有利益），那他就不会去过度使用公共牧场。

因此对D的理解是否应该如下：

D:If one user of common land overuses it even slightly, the other users are likely to do so even more, with the consequence that the costs to each user outweight the benefits.

假如一个放牧者即使稍微少一点过度使用公共牧场，那么其它放牧者会更少过度使用它，（因为）其它放牧者（会根据上一个放牧者减少过度使用公共牧场的行为)有一个推论（with the consequence)that：（上一个放牧者肯定是知道）在公共牧场放牧的成本高于获益（所以才减少过度使用公共牧场，那么我又为什么要去受害呢self-interest！）。

一般来说，rancher 更糟蹋common的草地，是因为他们可以从公共的草地中得到benefit。然而，这个由于过度使用草地造成草地质量下降的花费成本要分摊到每个rancher身上。结论是，公共草地仍然保护很好。从第一句话可以看出rancher之所以通常会滥用公共草地是因为能从中得到benefit，这也就是滥用common草地的前提。然而D选项说滥用的结果会造成花费超过benefit。而题目还有个前提是cost需要rancher们平摊，这样人们当然不会去糟蹋草地了。