求助0101的几道语法题，谢谢！

uspresidentUID: 2123602

:confused::(希望大家帮忙解答一下啊，谢谢谢谢！:)
6. by 1872 the United States had 70 engineering colleges, ------ astonishing expansion credited largely to the Morrill Act of 1862.
  (A) because
  (B) an
  (C) to which
  (D) was
为什么不能选A呢？不然不是谓语动词不专一了吗？谢谢！
8. 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
这道题为什么选D？始终搞不明白，我选的是A哪里不对？谢谢！
13. 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
为什么选C啊，怎么也读不通啊？我选D
28. The United States census (for) 1970 (showed that) the French-speaking residents of Louisiana (were) one of the country’s most compact regional linguistic (minority).
D错在哪里了呢？应该怎么改？谢谢！

charmyUID: 2156979

个人以为：
第一题选B，可以看作是前一分句的同位结构；也可以看作是which was an astonishing expansion credited largely to the Morrill Act of 1862. 的省略形式；选A后面的分句成分缺；
第二题，句子意思是有了催化剂后怎么样怎么样，in the presnce of which=with which，做状语，选A不能做状语，A如果是with whose presence也许就可以了；
第3题，whose指the point‘s，不放在定语从句里面应该是这样说的the point's sum of the point's distances  from two fixed points is constant;代换一下，就是C了；选D 是想把that做sum的同位语从句，但是很遗憾后面的不是句子（the distances from two points）;
最后一个， one of....用复数吧, minorities

uspresidentUID: 2123602

高手，谢谢！

yuhang3650UID: 2165177

请问楼上的，你的意思是13题印错了吗“point's ","points","points' ",到底是哪个啊

charmyUID: 2156979

没有印错，就是指的单一的点，
这么说可能更清楚点，the sum of the distances of the Point from two fixed points is constant.
可能单纯那样拆开来有点生硬，我理解的时候形象化了
就是把sum,和2个distance都看作为椭圆上任何一个点的性质(possessions)，
so用所有格的形式表示出来了

Ilove10UID: 203754

B
D
C 但是你的选项写错了
D one of 后面跟名词复数