I was about to give up my effort to solve a problem a good idea struck me.
A. when
B. while
C. until
D. before
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(答案→)A. when
B. while
C. until
D. before
解析：考查连词：AB都有“当…的时候”，when表示时间点，而且和be about to连用，表示“刚要做…这时”，while表示时间段，until直到，before在…前面，句意：我刚要放弃努力解决这个问题这时哦想到一个好主意。选A。