South Korean Foreign Minister Yoo Myung-hwan told lawmakers Tuesday joining PSI was the correct move for a "mature nation."

VOA: standard.2009.05.26

psi2 1 -1 We can re-write this as psi 2 1 negative 1.

我们可以重新写成。

麻省理工公开课 - 化学原理课程节选