Originally they were round pieces of metal with a hole in the center, so that a piece of string could keep them together.

最初它们是圆圆的金属片，中间有一个洞，这样一根绳子就可以把它们串在一起。

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Not only does it keep the city clean, the center provides income for thousands of poor people who are reimbursed for collecting plastic waste.

该回收中心不仅能使城市保持清洁，同时也给成千上万捡拾塑料垃圾的穷人带来了收益。

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The heated air molecules keep the object balanced in the dark center.

光线照射不到波束的中心，在那里微粒被受热的空气分子保持在平衡状态。

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They also keep watch on the American state of Hawaii from an operations center near Honolulu.

VOA: special.2009.03.17

So, we keep the atoms with the lowest ionization energy in the center.

因此，我们把电离能,最低的原子放在中间。

麻省理工公开课 - 化学原理课程节选

But it's a struggle to keep the center open.

VOA: standard.2010.04.29

In the past, she has sold her own jewelry and possessions to keep the center going.

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But, Kadima, the political party founded by the now ailing Prime Minister Ariel Sharon, has a chance to keep the political center alive in Israel and to hold onto the popularity it enjoyed when Mr. Sharon was in better health.

NPR: The Future of Sharon's Kadima Party

Key Republicans said the White House must keep Norwood at the center of its efforts if it hopes to pass a bill the president can support.

CNN: GOP maneuvering to push own patients' bill