"People who are traveling on a budget are not coming because they do not want to spend two times our tour price just on their flights."

VOA: standard.2010.06.03

And so there are only two kinds of waves that can exist on this violin string, standing or traveling.

由于这只有两种波,可以存在在这条弦上，驻波或行波。

麻省理工公开课 - 固态化学导论课程节选

And so there are only two kinds of waves that can exist on this violin string, standing or traveling.

由于这只有两种波,可以存在在这条弦上，驻波或行波。

麻省理工公开课 - 固态化学导论课程节选