At the time,Irma was a single mother, working at every opportunity to support her four children.

VOA: standard.2009.12.25

Remember, every time we have two electrons that are being shared, that's a single bond.

还记得，每次我们有两个电子被共用，我们就有了一个单键。

麻省理工公开课 - 化学原理课程节选