It was not until dark he found he thought was the correct way to solve the problem.A. th

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A. that; that
(答案→)B. that; what
C. when; what
D. when; that
解析：考查强调句型和宾语从句。本题第一空是强调句型，强调的是时间状语not until dark;第二空是动词find的宾语从句，what在句中做主语。句意：直到天黑他才发现了他认为是正确的解决这些问题的方法。故B正确。