他在黑暗中绊了一跤，差点摔倒。

He ___stumbled in the dark and almost fell.

她在演讲时结结巴巴，思路中断了。

She ___stumbled over her words during the speech, losing her train of thought.

我在整理旧照片时偶然发现了这张珍贵的合影。

I ___stumbled across this precious group photo while organizing old pictures.