【题目】By the time Jack returned home from England, his son .
A. graduated B. has graduated
C. had been graduating D. had graduated
在线课程【答案】D
【解析】
试题分析：考查时态。句意：杰克从英格兰回到家时，他的儿子已经大学毕业了。by the time...“到……的时候”引导时间状语从句时，如果从句的谓语动词用一般过去时，主句的谓语动词要用过去完成时，故选D项。
考点 : 考查时态