By the time Jack returned home from England, his son from college.
A. graduated
B. has graduated
C. had been graduating
D. had graduated
网友回答
A. graduated
B. has graduated
C. had been graduating
(答案→)D. had graduated
解析：句意：当杰克从英国回家为止，他的儿子已经从大学毕业了。by the time …当……的时候，……
通常引导一个时间状语从句，表示“到……的时候”主句则表示在此时间之前某个事件已完成。值得注意的是，当从句用过去时时，主句通常用过去完成时。选D。