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(E) works just as well as does a commonly used blood-thinning drug, which is also more expensive, at preventing blood clots
比较，对主谓宾结构的句型主语比较，需要补出助动词，注意助动词的形式、单复数及时态。
简洁有效，修饰名词以形容词最为简洁，尽量保持原句的修饰方式，由名词改为定语从句使得句子��嗦。
(A) 正确，aspirin与blood-thinning drug在prevent blood clots上的比较，does指代prevents，补出以避免歧义。
(B) a commonly used and a more expensive中的第二个冠词a多余，使得句意变化，成为两种drug。
(C) 助动词will应该为does，因为原句用的一般现在时prevents则不是将来时态。
(D) works at prevention of…的表达方式��嗦;缺少助动词does。
(E) works at preventing…的表达方式��嗦;which引导定语从句修饰drug��嗦，不如直接用形容词作定语。
补充说明：比较-主语比较
1、对主谓宾全的句型主语比较：A do sth than B do.(AB为对等名词，加do是为了显示这是一个可以发出动作的名词，因而确定是主语，进而确定是主语对比)。时态要据后定。There be 句型等同于一般的主谓宾俱全的句型。
主语若相同，可省，但助动词不能省。如：
As a result of the continuing decline in the birth rate, [fewer people] will enter the labor force [in the 1980’s+ than (few people省略)did *in the 1960’s and 1970’s+, a twenty-year period during which people born after the war swelled the ranks of workers.
2、对只有主谓(如果主语有定语成分，必须要that/those来指代)，没有宾语的句子，因为没有宾语，只可能有主语比较，因而不需要重复do来确认这是一个可发出动作的主语，所以其比较形式如下：
I eat faster than you.注意与下句区别 I eat apple faster than you do 如果写成 I eat apple faster than you 按GMAC的理解，从语法上就变成我吃苹果比吃你快，成了宾语比较。逻辑上显然不通。
3、主系表结构的比较与主谓结构相同。如果时态不一致，则需补出助词，如：
The economy of Florida is more diversified than it was ten years ago
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代词
代词pronoun简称pron是代替名词的一种词类。大多数代词具有名词和形容词的功能。
1. 代词的分类:
主格： I, you, she, he, it , we, they , who
宾格： me, you, him , her, it, us, them, whom
形容词性: this that those these
2. 代词的注意事项:
原则： 每一个代词都必须有一个确定的先行词。GMAT不适用带有性别的代词;所以仔细看代词 it/its/they/them/their.
原则： 每一个代词都必须在数量上和他的先行词保持一致
排除： 代词和名词在确定为名词是代词的逻辑语义后;数量上保持不一致的
原则： 当从句的主语是一个代词的时候，那么这个代词的先行词会是主句的主语;当有两个并列句的时候，第二句话的主语是一个代词，那么这个代词的先行词就是第一句话主语。
Eg. Because it was robbed last month, the door to the house is locked. 错误
“It” is the subject of the dependent clause, so it must be illogically referring to “door”
暗示： 当你注意到这些线索的时候，注意观察代词和先行词是否保持一致。代词问题;
当你每次看到一个代词的时候，必须要找到他的先行词;
3. 代词问题
3.1一致性问题
代词必须和前面的指代内容保持单复数，性别上的一致 指代对象是单数，单词是单数; 指代对象是复数，代词也是复数。
The Glass House Mountains in Queensland, Australia, were sighted in 1770 by the English navigator Captain James Cook, bv whom they were named supposedly because its sheer wet rocks glistened like glass.
① who so named them supposedly because their
② who so named it since supposedly their
答案是A 一个什么样的山(mountain),别人用他命名。。。。这里的it,或者them指代前面的mountains。所以要要复数them，而不能b中单数谓语it
2) In late 1997, the chambers inside the pyramid of the Pharaoh Menkaure at Giza were closed to visitors for cleaning and repair due to moisture exhaled bv tourists, which raised its humidity to such levels so that salt from the stone was crystallizing and fungus was growing on the walls.
a. due to moisture that tourists had exhaled, thereby raising its humidity to such levels that salt from the stone would crystallize
b. because moisture exhaled by tourists had raised the humidity within them to such levels that salt from the stone was crystallizing
