Question

仅仅一道英语选择。答案是B，不要答案，只要详解，为什么要这么做

They decided to cycle along the Mekong River from _____ it begins to ______ it ends.
A. what; what B. where; where C. the place where; what D. what; the place where

Answer 1

先把句子写完整:
They decided to cycle along the Mekong River from where it begins to where it ends.
=
They decided to cycle along the Mekong River from its source to its end.
from its source to its end ,是介词短语,做主句的状语.
回头看原句:
They decided to cycle along the Mekong River from where it begins to where it ends.
where it begins. where 引导的从句,where 做状语.= At the place, it begins. 同样,where it ends=At the place, it ends.

where it begins 是where(副词) 引导的名词从句,做介词from 的宾语,是宾语从句.
where it ends 是where 引导的宾语从句,做to 的宾语.
整个句子中,from where it ends to where it ends, 可以被看作from A to B, 做全句的状语.

A.选项,what,作为代词,无法在从句中做状语,不可引导从句, 排除.
C, 前部分可以,后不分不可以.D.情况与C相反.

from 后面接的不是定语从句，如下才是:
They decided to cycle along the Mekong River from the place(1) where it begins to the place (2) where it ends.
在这样的句子中,place 1,2 后where 引导的都是定语从句.而
from place(1) to place (2),在主句中,仍然是状语.
再进一步具体,
They decided to cycle along the Mekong River from Yunan where it begins to Vietnam where it ends.

the Mekong River 的定语从句应该是,
(the Mekong River) which begins in Yunnan and ends in Vietnam.

Answer 2

They decided to cycle along the Mekong River from _____ it begins to ______ it ends.
A. what; what B. where; where C. the place where; what D. what; the place where
这个句子，是【定语从句】中的一种特例。高考题中直接考过这一考点。答案是这样解释的：
当先行词表示地点，关系代词前面有介词from时,应该用where来引导定语从句。一是因为句意表达的需要【从......地方】；二是，这种使用习惯已经逐渐被当做【固定搭配结构】来对待。
祝你开心如意！O(∩_∩)O~~

Answer 3

第一个空where是因为where引导一个主语从句：where it begins;第二个where引导宾语从句：where it end .