Only after they had carried out over ten experiments the decision.A. they reachedB. they

问题补充：

Only after they had carried out over ten experiments the decision.

A. they reached

B. they reach

C. did they reach

D. do they reach

答案：

A. they reached

B. they reach

(答案→)C. did they reach

D. do they reach

解析：

这题考查部分倒装句的用法，当“only+状语”位于句首时，主句用部分倒装，即把助动词、情态动词、be动词放在主语前面，另外注意前面的过去完成时，横线处用过去时，所以选C。

句意是：只有在他们做了十个实验后，才下了决定。