Although the Supreme Court ruled as long ago as 1880 that Blacks could not be excluded outright from jury service, nearly a centry 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

(C) is necessary to develop and enforce the principle that all juries must (A)

请问这里为什么一定要用has been 而不能用is呢?

nearly a centry 是关键.