You really need that because if you have only one copy of something, you don't have a backup; so, you can store the one copy separately.

你真的很需要这项发明,因为要是你只有一份的话，你就没有备份;,这样的话，你就可以将副本单独留存。

耶鲁公开课 - 金融市场课程节选

Notice if I'm going to have two different sub problems I need to have two base cases here to catch this. And if I only had one it would error out. And as a consequence, I can go off and ask about rabbits. Let's see.

我需要定义两个基础事件来处理它,如果我只有一个基础事件的话就会产生错误,这样我就能够知道到底有多少只兔子了,让我们来看看，在第12个月末,没那么简单，在第二年末兔子就很多了。

麻省理工公开课 - 计算机科学及编程导论课程节选

If we have a double bond, we know we need to have only one sigma bond, and we're also going to have one pi bond.

如果我们有双键，我们知道我们需要一个sigma键，还需要一个π键。

麻省理工公开课 - 化学原理课程节选