声波在一种介质中顺畅地传播，突然触及了不同的介质，这时会发生什么？

You've got a sound wave traveling merrily along through one medium, when suddenly it hits a different medium, what does gonna happen then?

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这时，汤姆开始在写字板上潦草地写着什么，用手挡住不让那姑娘看见。

Now Tom began to scrawl something on the slate, hiding the words from the girl.

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她刚从大学毕业，在一家广告公司找到了一份工作，这时她开始感到她身体里正在发生什么奇怪的事情。

She has just graduated from college and got a job at an advertising agency when she began to sense that something strange was going on inside her body.

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So now, the enzyme is locked up with being built, and the penicillin has effectively killed the bacteria.

所以这是我第一次感受到,我想在单个原子的层面上思考这些分子,所以这时我的专业转到了化学方向。

麻省理工公开课 - 化学原理课程节选

It was a kind of powerful moment in American culture where the president allied himself with poetry in this way.

这在美国文化中是个伟大的时刻,这时总统用这种方法把自己和诗歌连接。

耶鲁公开课 - 现代诗歌课程节选

That is the created world, but he doesn't stop there And he creates various heavenly bodies at this point.

这便是他所创造的世界，可他却没有在此止步,这时他创造了各种各样的天体。

耶鲁公开课 - 旧约导论课程节选