Some anthropologists believe that the genetic homogeneity evident in the world's people is the result of a "population bottleneck"—at some time in the past our ancestors suffered an event, greatly reducing their numbers and thus our genetic variation.


at some time in the past our ancestors suffered an event, greatly reducing their numbers

that at some time in the past our ancestors suffered an event that greatly reduced their numbers

that some time in the past our ancestors suffered an event so that their numbers were greatly reduced,

some time in the past our ancestors suffered an event from which their numbers were greatly reduced

some time in the past, that our ancestors suffered an event so as to reduce their numbers greatly,

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考题讲解

题目分析：
破折号是一个很神奇的符号，身后的内容既可以修饰一个词，也可以修饰一个句子。在本题中，破折号身后加上用that引导的从句或者省略这个that都是可以接受的。

选项分析：

A选项：reducing their numbers是伴随状语，其逻辑主语应该为其所修饰的句子的主语，即，our ancestors suffered an event的主语。但在逻辑上，下降祖先数量的应该是事件，而不是祖先本身。

B选项：Correct. 本选项在语法和逻辑上均是正确的。

C选项：so that their numbers were greatly reduced是结果状语从句，但其是被动语态。我们可以认为其在语义上约等于：
某人或某物下降了我们祖先的数量。
联系语境则有：
我们的祖先遭受到了一个事件，如此以至于，某人或某物下降了我们祖先的数量。
划线的两句话之间没有了因果关系。（肯定应该是，遭受事件，如此以至于，这件事下降了数量）
另外，划线部分身后的and thus our genetic variation需要和并列连词and身前的内容屏行，即，reduce their numbers and thus our genetic variation。

D选项：本选项中定语从句补全为：
their numbers were greatly reduced from an event.
这是不合逻辑的，例如：
(1) 在中国，这有一辆车。（In China, there is a car.）
(2) 中国有一辆车。（China has a car） 
两个例句的意思是完全不同的。句(1)中，我们无法确定这辆车是否属于中国所有，例如这辆车的主人是个美国人，他把车放在了中国境内，但你不能说，这辆车就是中国的。用之于本题，必须是这个事件下降了祖先的数量。否则不能说祖先们suffer了这个事件。

E选项：so as to身后的动词的主语必须是其主句的主语。在本选项中是our ancestors。但在逻辑上，下降了祖先数量的应该是“事件（an event）”，而不是祖先自己。另外，引导词that应该放在sometime in the past身前。

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