故事发生在大约半个世纪以前。
然而，第一个慢性中毒病例发生在半个世纪以前。
However, the first chronic case took place half a century ago.
半个世纪以前很难想象去月亮上旅行。
Half a century ago it's difficult to conceive of traveling to the moon.
半个世纪以前很难想象去月亮上旅行。
Half a century ago it's difficult to conceive of traveling to the moon.
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