OG12 43 but用法 题目已可见

?? Laos has a land area about the same as Great Britain
but only four million in population, where many are
members of hill tribes ensconced in the virtually
inaccessible mountain valleys of the north.
?? (A) about the same as Great Britain but only four million
in population, where many
?? (B) of about the same size as Great Britain is, but in
Laos there is a population of only four million, and many
?? (C) that is about the same size as Great Britain’s land
area, but in Laos with a population of only four million
people, many of them
?? (D) comparable to the size of Great Britain, but only four
million in population, and many
?? (E) comparable to that of Great Britain but a population
of only four million people, many of whom

正确答案为E，而且我不是很明白D为什么错？。。。
还有一个就是我想问下，是否在为划线部分未出现过代词，其他选项中也没有代词的情况下，某个选项出现代词基本都会犯指代不明的错误？。。。。