做多了 GRE 阅读的同学应该已经总结出来，阅读多数段落的主旨句都在一些很典型的位置。
最常见的写法是开门见山，开头即是主旨。
如果出现在中间也往往伴随典型的关键词给与读者足够的提示。
但在阅读的一篇高频文章中，有一段话的主旨出现在了一个表面看起来平平无奇的位置，因此导致关于这段话一道宏观题目的大规模错误。
这个段落全文如下：
①The Earth-Moon system has always been perplexing. ②Earth is the only one of the inner planets with a large satellite, the orbit of which is neither in the equatorial plane of Earth nor in the plane in which the other planets lie. ③The Moon’s mean density is much lower than that of Earth but is about the same as that of Earth’s mantle. ④This similarity in density has long prompted speculation that the Moon split away from a rapidly rotating Earth, but this idea founders on two observations. ⑤In order to spin off the Moon, Earth would have had to rotate so fast that a day would have lasted less than three hours. ⑥Science offers no plausible explanation of how it could have slowed to its current rotational rate from that speed. ⑦Moreover, the Moon’s composition, though similar to that of Earth’s mantle, is not a precise match. ⑧Theorizing a titanic collision eliminates postulating a too-rapidly spinning Earth and accounts for the Moon’s peculiar composition. ⑨In a titanic collision model, the bulk of the Moon would have formed from a combination of material from the impactor and Earth’s mantle. ⑩Most of the earthly component would have been in the form of melted or vaporized matter. ⑪The difficulty in recondensing this vapor in Earth’s orbit, and its subsequent loss to the vacuum of outer space, might account for the observed absence in lunar rocks of certain readily vaporized compounds and elements.
由于这段话的开头的句子读起来非常有概括性，很有主旨句的感觉，很多同学非常武断得认为主旨句是第一句。
但仔细往后读会发现，只有②③两句是围绕第一句的“地月关系很复杂”展开。
另一个可以提示第一句不是主旨句的思路是，第一句是典型的事实陈述，而阅读往往以观点为重。
接下来有些同学可能会认为主旨句是第④句，因为提出了 speculation即一种解释地月形成的理论——地球快速旋转，甩出去形成月球。
后半句表明这个观点是错误的this ideas founders on two observations也很有概括性。但这句所统领的 two observations 在⑤到⑦句即陈述完毕。
第⑦句的 moreover 也可以提示之前是第一点，第⑦句本身则是第二点。
这段话实际的主旨句是第⑧句。这句话作为主旨句来说在阅读的段落里是非常少见的类型，出现在段落的中间且没有出现转折词或小结词。
也正是因此而使得很多同学会忽略掉这个重要句。这句话承上启下，既总结了前半段的难题在于快速旋转学说（fast splitting）无法解释很多问题，又开启了下半段为什么titanic collision这个理论好。
在理解了这段话的实际结构与主旨后，这道题目你能做对吗？
In the second paragraph, the author is primarily concerned with
A. arguing in favor of a particular theory about the formation of the Earth-Moon system.
B. summarizing conventional theories about the formation of the earth-Moon system.
C. anticipating and responding to criticisms of a particular theory about the formation of the Earth-Moon system.
D. explaining why the Earth-Moon system is considered scientifically perplexing.
E. questioning an assumption underlying one theory about the formation of the Earth-Moon system.
答案：A。
解析：对应第8句，整段主旨是为了支持新观点 titanic collision 理论。