By the time Jack returned home from America, his son from college.
A. graduate
B. has graduated
C. had graduated
D. had been graduated
网友回答
A. graduate
B. has graduated
(答案→)C. had graduated
D. had been graduated
解析：考查时态。By与完成时连用，本题by后面是表示过去的时间，所以使用过去完成时。句意：到JACK回到美国的时候，他儿子已经大学毕业了。