请教!!定从的问题

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 the sum of whose
D whose sum that the                                                                              

答案选C, 这样好像跟后面的is不一致, 能解释一下句子结构吗?

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                                                                        

答案是D,不太明白,请教大家

1，the subject of "is" is "the sum"

whose=any point

2,which=chlorophyll

in the presence of chlorophyll --------从句状语=where

以下是引用consonant在2005-4-22 20:22:00的发言：

1，the subject of "is" is "the sum"

whose=any point

2,which=chlorophyll

in the presence of chlorophyll --------从句状语=where

这两个定从的句子都挺怪,先行词与其他词组成短语在从句充当成分,挺晕的

请教一下应该如何去做这样的题?

把握住句子结构就不难了。

糊涂的时候可采用排除法。