Researchers testing birds in the Northeast have found creeping mercury levels in the blood of more than 175 once clean species.

在东北用鸟类做试验的研究者在175种曾经列为安全鸟类的血液中发现水银。

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The interior blends Mr. Simonson's original design with new and cleverly repurposed materials, creating a look at once clean and rustic.

内饰融合了西蒙森的原始设计和巧妙改变用途的新材料，第一眼就给人以干净质朴的观感。

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Must we clean the classroom at once?

我们必须马上打扫教室吗？

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Many generalist venture capital firms that once had clean-tech practices have stopped making new investments or exited altogether.

FORBES: After A Bad Quarter Here's The New Rules For Green Tech Investing

As a final nail in the coffin of his once-clean image, Oakley dropped Mr. Armstrong immediately upon hearing the news from the International Cycling Union.

FORBES: Stripped Tour Titles Strips Lance Armstrong of Brand Significance

Once you have done so, it's time to wipe that slate clean once and for all.

CNN: Feel the relief of a Facebook purge