大神们请教个主旨题。。。！！！！

专注GRE三十年UID: 3277319

Diamonds, an occasional component of rare igneous rocks called lamproites and kimberlites, have never been dated satisfactorily. However, some diamonds contain minute inclusions of silicate minerals, commonly olivine, pyroxene, and garnet. These minerals can be dated by radioactive decay techniques because of the very small quantities of radioactive trace elements they, in turn, contain. Usually, it is possible to conclude that the inclusions are older than their diamond hosts, but with little indication of the time interval involved. Sometimes, however, the crystal form of the silicate inclusions is observed to resemble more closely the internal structure of diamond than that of other silicate minerals. It is not known how rare this resemblance is, or whether it is most often seen in inclusions of silicates such as garnet, whose crystallography is generally somewhat similar to that of diamond; but when present, the resemblance is regarded as compelling evidence that the diamonds and inclusions are truly cogenetic.
20.        The main purpose of the passage is to
(A) explain why it has not been possible to determine the age of diamonds
(B) explain how it might be possible to date some diamonds
(C) compare two alternative approaches to determining the age of diamonds=
(D) compare a method of dating diamonds with a method used to date certain silicate minerals
(E) compare the age of diamonds with that of certain silicate minerals contained within them
我选了C~~答案是B？为什么呢？B不太理解~~

chenshinjiUID: 3308264

第一句说钻石不能被鉴定时间。第二句However表明作者要进行转折，也就是否定第一句话的观点。文章下面的内容就是介绍两种可能鉴定时间的方法。这两种方法是用来支持作者反驳钻石不能被鉴时间的，所以不是主旨，主旨是钻石可能被鉴定。

专注GRE三十年UID: 3277319

chenshinji 发表于 2012-7-20 16:14
第一句说钻石不能被鉴定时间。第二句However表明作者要进行转折，也就是否定第一句话的观点。文章下面的内容 ...

通透了！谢谢！