OG12 SC43老题新问，菜鸟求NN们解救！

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43.Laos has a land area about the same as Great Britainbut only four million inpopulation, where many aremembers of hill tribes ensconced in the virtually inaccessible mountain valleys of the north.

(A)about the same as Great Britain but only fourmillion in population, where many

(B)of about the same size as Great Britain is, but inLaos there is a population of only four million,and many

（C）that is about the same size as Great Britain'sland area, but in Laos with a population of onlyfour million people, many of them

(D)comparable to the size of Great Britain, but onlyfour million in population, and many

(E)comparable to that of Great Britain but apopulation of only four million people, many of whom

答案是E。看了之前的帖子，大家对C的解释是but后面不是完整句。但是菜鸟我的问题是，难道不可以把but后面的内容看作是：but + 介词短语（做状语），+clause2 吗？这样不就是完整句了么？介词短语修饰clause2。
还有不能用them，为什么them不能指代people呢？为什么说them指代不清呢？
菜鸟级别，求解救！！！第一遍看OG，问题多多。求NN指导！
跪谢！