4 SC请教

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1. (At the invitation of) Kwame Nkrumah, W.E.B.Du Bois settled in Gharla and became the editor of the Encyclopedia Afticana.
a.
b. When he was invited by
c. An invitation coming form
d. An invitation that came from
e. After having an invitation by

2. Dr. Sayre's lecture recounted several little-known episodes in the (relations between nations that illustrates) what is wrong with alliances and treaties that do not have popluar support.
a.
b. relation of one nation with another that illustrates
c. relations between nations that illustrate
d. relation of one nation with another and illustrate
e. relations of nations that illustrates

3. (A substance derived from the Madagascar periwinkle, which has proved useful in decreasing mortality among young leukemia patients,) is cultivated in China as part of a program to integrate traditional gerbal medicine into a contemporary system of health care.
a.
c. A Madagascar periwinkle derivative, which has proved useful in decreasing mortality among young leukemia patients,
e. The Madagascar periwinkle, a derivative of which has proved useful in decreasing mortality among young leukemia patients

4. With diamonds, as with all gems, (one should ask for a written description of one's purchase;) the description may prove useful later if you have reason to believe the jeweler misled you.
a.
b.you should ask for a written description of your purchase
e. a written description of your purchase is what should be asked for

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Forget the given answers to those SC.
1. A, 2.B, 3. E, 4. B

不是很明白这些选项为什么正确．

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THANKS!!
No.2: 是不是两个以上用AMONG,两个及以下用BETWEEN? 例如:The difference between these 2 guys is obvious. The difference among those three guys is obvious.
BTW, 你门是怎么一眼看出NATION和TREATY搭配不对?我一上来看到BTWEEN就去找AND,结果上当受骗.

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3. 为什么derivative延生物,不能被cultivated??
4. 因为句首有WITH　ＤＩＡＭＯＮＤ，　我怎么觉得和a written description （而不是正确答案里的ＹＯＵ）搭配更好呢？