Only after they had discussed the matter for several hours a decision.
A. they reached
B. did they reach
C. they reach
D. do they reach
网友回答
A. they reached
(答案→)B. did they reach
C. they reach
D. do they reach
解析：考查倒装句。only+状语或状语从句位于句首，主谓用部分倒装。故排除A与C。又因为时态应前后一致。故选B。