Not until the diffiult problem.A. did he return;we settledB. he returned;we settledC. did

问题补充：

Not until ,the diffiult problem.

A. did he return;we settled

B. he returned;we settled

C. did he return,did we settle

D. he returned;did we settle.

答案：

A. did he return;we settled

B. he returned;we settled

C. did he return,did we settle

(答案→)D. he returned;did we settle.

解析：本题考查倒装。Not until位于句首时，主句要用部分倒装，但until后的从句不倒装。