请教一题

1.       
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（A）

(E) will be
necessary for developing and enforcing the principle of all juries being

 答案就不说了，主要是a和d的区别。a表示一件事发生在过去，一直持续到现在，并且有可能继续发展下去。d表示现在是这样，没有对历史的表示。