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：本题考查动词时态。句意为“当杰克从英国返回家时，他的儿子已经大学毕业了。”by the time到…时候为止，一般与完成时连用。选D。