It was not until dark _____ he found _____ he thought was the correct way to solve the problem. [ ]

A. that; what

B. that; that

C. when; what

D. when; that

斗轮机夹轮器电气回路中设置了限位开关，通过电气连锁，保证只有在夹钳松开时才能启动行走机构。