It was not until dark he found that it was the correct way to solve the problem.A. thatB

问题补充：

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

答案：

(答案→)A. that

B. what

C. which

D. when

解析：强调句型，句意：直到天黑，他才发现这是一个正确的方法来解决这个问题。故选A.