By the time Jack returned home from America his son from college.A. graduateB. has gradu

问题补充：

By the time Jack returned home from America, his son from college.

A. graduate

B. has graduated

C. had graduated

D. had been graduated

答案：

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。