The urge to go abroad for further study is strong but it was not until last summer I kne

问题补充：

The urge to go abroad for further study is strong, but it was not until last summer I knew how powerful it could be.

A. who

B. that

C. where

D. before

答案：

A. who

(答案→)B. that

C. where

D. before

解析：强调句用来引起他人注意你所谈的内容，其结构是：“It is/was＋被强调的部分＋that/who＋句子剩下的部分”。分析句子结构可知，but后面的分句是强调句结构，被强调的部分是时间状语，故答案选B。