请教885-2-15 ,一个讨论过，但没有讨论清楚的问题。

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

答案是A,可为什么不能是D呢？

觉得原来的讨论不清楚，需要继续讨论。

是不是因为（类似的）从句中用过去时，则主句若用现在时时，一般需用现在完成时？

那还是因为你对时间状语所对应的时态不太了解.多看一些基本的语法书,你就知道了.

就是说，凡是nearly a century的情况，都用完成时态，不可用现在时？