Do they only belong and serve for certain people in certain time?

它们是否只存在于某些特定时间，服务于特定人群？

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The classroom teaching is conducted in certain time and limited space.

课堂教学是在一定的时间和有限的空间中开展的。

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Second, in certain facilities, a large number of people use it in certain time periods, while few of people use it in other time periods.

第二，在部分场馆，有些时间段使用人数较多，而其他时间段使用人数较少甚至闲置。

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The bill would free all slaves in the district who were born after a certain time.

VOA: special.2009.03.12

It began to infiltrate other disciplines and was in general a high-spirited horse for that certain period of time.

它逐渐渗入到其它学科,总之是那个时代的一匹黑马。

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It's actually better to spend a certain amount of time just keeping your body in good condition

其实花一定时间保持身体的良好状态更好，

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At the same time, due to the crisis and to the difficulties which we had in our relations a certain time ago, this potential has not been tapped.

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About two months ago Regal-Beloits Durst power transmission division in Beloit bailed out a longtime customer when the customers supplier couldnt deliver a certain axle shaft in time.

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Shareholders, unlike bondholders, receive no legal promise to be repaid in cash at a certain time.

ECONOMIST: Stocks in trade | The