我在机场偶然遇到了一位老朋友。

I met an old friend ___by ___chance at the airport.

很凑巧，我需要的那本书正在打折。

___By ___chance, the book I needed was on sale.

我们很偶然地找到了那套完美的公寓。

We found the perfect apartment ___by ___chance.