请教XDF的几道题目

zjxmlily

15. Although the Supreme Court ruled as long ago as 1880 that Blacks could not be excluded outright from jury service, nearly a century of case-by-case adjudication has been necessary to develop and enforce the principle that all juries must be drawn  from “a fair cross section of the community.”
(A) has been necessary to develop and enforce the
   principle that all juries must be
(B) was necessary for developing and enforcing  the principle of all juries being
(C) was to be necessary in developing and enforcing the principle of all juries to be
(D) is necessary to develop and enforce the principle that all juries must be
(E) will be necessary for developing and enforcing the principle of all juries being
the key is A,why not choose D?
请问一般时态和现在完成时态应该分别运用在哪些方面呢？为何这一题要用完成时态呢？
类似这种时态判断的还有一题也不是太理解，如下：
23. According to United States Air Force officials, a cannon shooting dead chickens at airplanes has proved helpful to demonstrate what kind of damage can result when jets fly into a flock of large birds.
(A) shooting dead chickens at airplanes has proved helpful to demonstrate
(B) shooting dead chickens at airplanes has proved itself helpful as a demonstration of
(C) shooting dead chickens at airplanes proves itself helpfull as demonstrating
(D) that shoots dead chickens at airplanes proves itself helpful to demonstrate
(E) that shoots dead chickens at airplanes has proved helpful in demonstrating
the key is E ,I choose D
另外还有一道是
10. For many people, household labor remains demanding even if able to afford household appliances their grandparents would find a miracle.
(A) even if able to afford household appliances their grandparents would find a miracle
(B) despite being able to afford household appliances their grandparents would find a miracle
(C) even if they can afford household appliances their grandparents would have found miraculous
(D) although they could afford household appliances their grandparents would find miraculous
(E) even if they are able to afford household appliances which would have been a miracle to their grandparents
the key is C, I choose D
C中为何要用would have呢？

谢谢！

zjxmlily

刚刚看了一个帖子才发现我没把出处写全，抱歉
上面三道题的具体出处是XDF SET2 15题，XDF SET3 23题和XDF SET3 10题
另外第一道我在另外一个帖子已经看到答案了，是不是说如果有出现一个持续时间的词或短语而且又没有明确的过去时态的词出现，就用现在完成时，否则就用过去完成时呢？
那第二道题目又为什么用完成时而不用一般时呢？

[此贴子已经被作者于2003-7-22 17:21:08编辑过]