By the time Jack returned home from England,his son___ from college.A.graduated B.has graduated C.had been graduating D.had graduated

Question

By the time Jack returned home from England,his son___ from college.
A.graduated B.has graduated C.had been graduating D.had graduated

Answer 1

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.
例如：
1.By the time l got to the station,the train had
already gone.
2.By the time l got home,they had already left.
基本上用法只有注意这些了,用惯了就会了