In a speech at Carnegie Mellon University, President Obama referred to the Gulf oil spill in making the case that it is time, as he put it, to "aggressively accelerate the nation's transition to a clean energy economy."

VOA: standard.2010.06.14

You spend a lot of time talking about the transition between alchemy and modern chemistry.

在高中化学你很多时间是花在,了解从炼金术到化学的改变上。

麻省理工公开课 - 化学原理课程节选