They marched out singly or in pairs.

他们一个一个地或成双成对地走出去。

They always come in pairs.

他们总是成对地出现。

These birds fly in pairs.

这些鸟成双成对地飞。

Students worked in pairs on the project.

学生两人一组做这个项目。

They are all sold in pairs.

他们都成对卖。