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The phone comes into play when there are multiple options to consider or important decisions to be made.

当有多种选择要考虑，或者有重要的决定要做时，手机就开始发挥作用了。

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These problems may appear and then disappear, sometimes resulting in long periods when there are no problems at all.

VOA: special.2009.01.06

So that's a big help. And equilibrium happens when there isn't any possible change of state that would satisfy this.

这个非常方便，当所有的变化,都无法满足这个公式的时候,系统就达到了平衡。

麻省理工公开课 - 热力学与动力学课程节选

which you'll think, only a ten or eleven year old, but when there is thirteen of them, it's a lot.

你会想，不就是十岁、十一岁的小孩嘛，但有十三个还是很多的。

大城市是否危险？ - SpeakingMax英语口语达人

Ms. SETZFAND: People actually look for information when there's a problem, when there's a pressure point.

NPR: Amid Meltdown, Retirement Ads Stay On Message

For example, back in March, when there was the biggest oil spill in Prudhoe Bay history.

NPR: Alaskans Speak Up About Oil Field Shutdown

There was a period of time when there wasn't much emphasis put on antibiotics.

NPR: Tracking a New Strain of Drug-Resistant Staph