请教如何确定时态

echochen

大全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

大全 256

      During an era when interracial tensions in the United States have run high, Rosa Parks became a quiet, unassuming symbol of the continued struggle for human dignity.

(A) have run high

(B) ran high

(C) had run highly

(D) run high

(E) were running highly

上述两例红颜色为正确答案,绿颜色为我选的答案.错误都出在时态上.请问NN如何确定正确的时态.

我的理解:第一题中nearly a century of case-by-case adjudication 大段的时间后应用完成时

              第二题中因为主句中became故从句也应用过去时.但是我们也经常能见到一句中出现时态混合现象的.

jnlvo

我简编了大全注释，在置顶的best of best中有，你可先参看一下，可能能帮助你解决些问题，如有还不清楚，再问。

dashasha

嗯？难道我色盲？没看出来颜色区别啊。答案是什么？A，B？

echochen

不好意思,没注意递交了以后颜色没了,第一题正确答案A,我选的是D

第二题正确答案为B我选了A