It was not until dark he found that it was the correct way to solve the problem.
A. that
B. what
C. which
D. when
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(答案→)A. that
B. what
C. which
D. when
解析：考查强调句的特殊句型：It’s not until ….that…,直到…才…,值得注意的是not until不是在句首，后面不能倒装，句意：直到天黑他才知道这是解决问题的正确的办法。选A。