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题库 / OG2017V-CR-167
Colorless diamonds can command high prices as gemstones. A type of less valuable diamonds can be treated to remove all color. Only sophisticated tests can distinguish such treated diamonds from naturally colorless ones. However,only 2 percent of diamonds mined are of the colored type that can be successfully treated, and many of those are of insufficient quality to make the treatment worthwhile. Surely, therefore, the vast majority of colorless diamonds sold by jewelers are naturally colorless.
A serious flaw in the reasoning of the argument is that
comparisons between the price diamonds command as gemstones and their value for other uses are omitted
information about the rarity of treated diamonds is not combined with information about the rarity of naturally colorless, gemstone diamonds
the possibility that colored diamonds might be used as gemstones, even without having been treated, is ignored
the currently available method for making colorless diamonds from colored ones is treated as though it were the only possible method for doing so
the difficulty that a customer of a jeweler would have in distinguishing a naturally colorless diamond from a treated one is not taken into account
B 稀有程度没有指出。如果自然无色的概率是0.0001%那么有色的概率99.9999%×2%远大于自然无色的概率 削弱文段
如果反过来加强文段
要削弱，就需要考虑这个2%是不是有问题，只说了带颜色的只有2%，但是没说colorless且成色好的原石的比例是多少，如果后者的比例更低，那加工带颜色的原石的动机就很充分了。
本题是一个统计枚举类的题 要注意single不等于relative
C: 售卖的无色钻石, most是天然的——天然钻石占无色钻石的大部分
gap: 如果 天然无色钻石占总钻石的比例
only 2 percent of diamonds mined钻石矿里只有2%是假冒钻石
注意：1.分母是所有钻石矿
2.有分数，注意考虑相对量和绝对量
P：假冒钻石只占总钻石的2%（比例小）
C：大部分无色钻石都是真的
Gap：可能真无色的比例也很小，也许远＜2%，那么大部分钻石反而是加的
所以gap在于，没有比较 假冒钻石和 真无色钻石的数量
因为2%的分母是所有钻石，而不是无色钻石
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没有同时考虑treated diamonds和colorless, gemstone diamonds的稀缺，文章只考虑了treated diamonds的稀缺，如果colorless, gemstone diamonds同样稀缺，那么，柜台售卖的不一定大部分都是colorless, gemstone diamonds
C项，直接在反前提
C项，直接在反前提
P：2%的有色钻可以被加工（因）
C：大部分的市售无色钻石都是真的（果）
直接反驳结论