So in one case, we could actually put carbon in the middle, in one place, we could put sulfur in the middle, and in one case we could put nitrogen.

那么有一种情况是，我们把碳放在中间，还有一种情况是，把硫放在中间，另外一种情况是把氮放在中间。

麻省理工公开课 - 化学原理课程节选