Only after they had discussed the matter for several hours a decision.　　A they reached B d

问题补充：

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

答案：

解析：本题考察only前置的倒装句。Only引导状语从句前置，主句主谓要部分倒装。又因为从前文中的had discussed判断出是过去时间，故选B；