选项
A.
they discussed the case
B.
they had discussed the case
C.
the case was discussed
D.
the case had been discussed
正确答案：A
名词解析
结构应用。现在分词短语作状语时，其逻辑主语应与句中主语一致。句中只能是they在discuss，分词逻辑主语是they，句子主语也应是they；所以选项c、D均不对；而前半句用的是现在分词的完成式，表示动作发生在谓语动词表示的动作之前，所以选项A是答案。