And in sub-Saharan Africa, where only four percent of the land is irrigated, buying a small pump can make a big difference.

VOA: standard.2010.08.09

If you want to have an infinitestimally small change, you end up writing dh is dq sub p.

这是对于有限的变化而言的,如果是一个无限小的变化。

麻省理工公开课 - 热力学与动力学课程节选

"For farmers in sub-Saharan Africa who are subsisting on very little money, a small reduction in price based on American subsidization is a big deal to them."

VOA: standard.2010.04.28