Question

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

Answer 1

C

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。