But traditional guidelines say not to use tPA if more than three hours have passed after the first signs of a stroke.

VOA: special.2009.08.12

And that's why, when I put up those three different reactions, and we saw the signs could vary.

这就是为什么，我写出了三个方程，但是能量和熵的符号却不相同。

麻省理工公开课 - 热力学与动力学课程节选

A new study shows that some obese children as young as three have early warning signs for heart disease.

VOA: standard.2010.03.09