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　　定语从句分为2种，限制性定语从句和非限制性定语从句。限定性定语从句对先行词起限制、确定作用,与先行词有着不可分割的联系,它的前面通常没有逗号;非限定性定语从句与它的先行词之间只有比较松散的关系,通常用逗号将它与主句隔开。

　　1.限制性定语从句：对所修饰的名词加以限制，表示“...的人(或东西)”The girl who sits in the chair is my sister.(who sits in the chair)是限制性定语从句，相当于一个形容词，修饰the girl.The girl whom I saw told me to come back today.The car that I hired broke down.

　　2.非限制性定语从句：对所修饰的次补充一些说明，通常有逗号隔开。Peter, who had been driving all day, suggested stopping at the next town.The 10:00 train, which is usually very punctual, was late today.

　　例题讲解：

　　More than 300 rivers drain into Siberia's Lake Baikal, which holds 20 percent of the world's fresh water, more than all the North American Great Lakes combined.

　　(A) More than 300 rivers drain into Siberia's Lake Baikal, which holds 20 percent of the world's fresh water, more than all the North American Great Lakes combined.

　　(B) With 20 percent of the world's fresh water, that is more than all the North American Great Lakes combined, Siberia's Lake Baikal has more than 300 rivers that drain into it.

　　(C) Siberia's Lake Baikal, with more than 300 rivers draining into it, it holds more of the world's fresh water than all that of the North American Great Lakes combined, 20 percent.

　　(D) While more than 300 rivers drain into it, Siberia's Lake Baikal holds 20 percent of the world's fresh water, which is more than all the North American Great Lakes combined.

　　(E) More than all the North American Great Lakes combined, Siberia's Lake Baikal, with more than 300 rivers draining into it, holds 20 percent of the world's fresh water.

　　考点: 比较(Comparison) 简洁有效(Rhetorical Construction)

　　1) 限定性从句和非限定性从句: 限定性定语从句对先行词起限制、确定作用,与先行词有着不可分割的联系,它的前面通常没有逗号;非限定性定语从句与它的先行词之间只有比较松散的关系,通常用逗号将它与主句隔开

　　2) 比较的双方必须概念对等

　　(A) 正确, 非限定定语从句which holds…就近修饰Lake Baikal; more than all the…作为20percent of the world’s fresh water的同位语,意思清晰,结构简单

　　(B) that引导限定性定语从句前面不用逗号隔开, 而此处去掉that从句并不影响原句的主要意思,因此应该用which引导的非限定性定语从句表明两者之间松散的逻辑关系, 但即使改为which引导的非限定性定语从句,which也只能修饰fresh water仍然意思不对; has more than 300 rivers that drain into it表达笨拙,不如A的结构简单

　　(C) 去掉插入语with more than 300…原句变成Siberia’s Lake Baikal it holds 20 percent… it多余.后半句结构混乱.

　　(D) while作为连词表示当…的时候或者表示转折,而原句并没有这种逻辑关系; which引导的非限定性定语从句就近修饰fresh water, 而从逻辑上应该修饰20 percent of the world’s fresh water

　　(E)将Baikal和北美5大湖直接进行比较,概念不对等, 应该是Baikal的水量和五大湖的水量进行比较.

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