我对这三道从句题特别晕.唉.

我对这三道从句题特别晕.唉.
The most important chemical catalyst on this planet is chlorophyll,-------carbon dioxide and water react to form carbohydrates.
(A) whose presence
(B) which is present
(C) presenting
(D) in the presence of which
Key：D
那个which到底指代的是什么啊,为什么选D,空格后面的句子不是主谓宾都全了么.
In geometry,an ellipse may be defined as the locus of all points -------distances from two fixed points is constant.
(A) which as the sum of
(B) of the sum which
(C) whose sum of whose
(D) whose sum that the
Key：A
到底怎么区分如何选用whose 和 which A里面为什么有个as?如果把C里面第二个whose去掉对不对?
An image on a national flag can symbolize political ideals that -------express.
(A) take many words to otherwise would.
(B) would take to many otherwise words
(C) many words to take would otherwise
（D）would otherwise take many words to
Key：D
otherwise是副词怎么在take的前面啊?为什么用would啊?其什么作用?

The most important chemical catalyst on this planet is chlorophyll,-------carbon dioxide and water react to form carbohydrates.
主句化简：The chemical catalyst is chlorophyll 化学催化剂是叶绿素
定语从句：in the presence of which carbon dioxide and water react to form carbohydrates；
which指代chlorophyll,in the presence of which 是从句的条件状语；从句的意思是“在它（叶绿素）存在的情况下二氧化物和水对形成碳水化合物起到了作用”
选项必须表示符合“在叶绿素存在的情况/前提下”这个要求
(A) whose presence：缺少介词in,只表示它的存在,不符合要求.
(B) which is present：填入从句不仅出现is和react两个动词,重要的是句子根本说不通,
(C) presenting：present作为出现、存在只能是形容词,不能用作动词,因此也没有分词形式 ；
(D) in the presence of which 符合要求.
In geometry,an ellipse may be defined as the locus of all points -------distances from two fixed points is constant.
主句：几何学中椭圆可以定义为所有点的轨迹
从句：
作用：点的轨迹 （locus of all points）也可以是直线和不封闭曲线,所以必须限定points的条件定义才能成立,所以必须要给locus加一个限定性定语从句,从句中的应该用which来指代locus这个先行词.
主要成分：which (即 the locus of all points) is constant “它（点的轨迹）是连续不断的”,
附加成分：介词短语 as the sum of distances from two fixed points “作为从两个确定点的距离的和”
基于上述分析,才能的达到椭圆的准确定义：“几何学中椭圆可以定义为作为从两个确定点的距离的和而连续不断的所有点的轨迹”.排除了直线和其他曲线.
答疑：whose在口语化的应用中可以用来代替which,即：whose ...= ...of which,所以上面的定语从句如果改为 whose locus of all points语法也是正确的,但是作为科学定义应该回避这种通俗说法.同样,第一题中的in the presence of which 也可以用 in whose presence 取代.
An image on a national flag can symbolize political ideals that -------express.
that从句：
1.关系代词that 指代 image,并在从句中作主语；
2.谓语部分：would take many words to express 会需要许多表达言辞
3.otherwise是副词,意思是“以其他方式”
从句翻译：这些象征会需要许多以其他方式表达的言辞.
(