[求助]大全101（已解答）

101. 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

本题的答案是A。

我的问题是，为何一定要用完成时？为何不能选D？

这里"nearly a century of case-by-case adjudication "是做什么成份,好象不是主语. 后面的不定试应该做主语.

这里是不是缺个it?

请NN讲讲.

谢谢!!!

主语中有a century 含有一段时间的意思，所以后面用了完成时。

个人见解是：
"nearly a century of case-by-case adjudication"做主语。

比如，我造个句子：A notebook PC is necessary to study in the business school in United States.

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