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
网友回答
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。