The answer to this riddle lies in the metabolic activity of early photosynthetic life-forms that slowly but surely transformed the chemical composition of Earth's atmosphere.

这个谜题的答案在于早期光合生命形式的新陈代谢活动，这种活动缓慢但一定改变了地球大气的化学成分。

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Scientists may now have an answer to this riddle, thanks to a study that cracks open one of the long-standing mysteries of getting a good night's sleep.

现在，科学家们也许找到了这个谜题的答案。当然，我们首先要感谢一项最终揭开了“如何获得一个好的夜晚睡眠”存在了很长时间的神秘问题谜底的研究。

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What she was hoping for was that the answer would unriddle the riddle, and bring rest and peace to her perplexed little mind.

她所希望的是妈妈的回答能够解决这个难解之谜，能够为她那困惑的小小心灵带来平静和安宁。

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Egoyan aims his restrained camera at a small Adirondack community that's suffering an answer to that riddle on a grand scale.

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His answer to that riddle has been to accuse Clinton and Vice President Al Gore of squandering the opportunity these good times offer to tackle fundamental long-term problems.

CNN: latimes.com: Bush the son uses policy campaign to distance himself from the father

We are still fishing for the answer to that riddle.

CNN: Commentary: Is it 1929 all over again?