这是最难回答的一个问题，因为从接触污染物到生病，通常会间隔很长时间。
This has been one of the hardest questions to answer because the time lag between exposure to pollutants and developing an illness is usually long.
这是最难回答的一个问题，因为从接触污染物到生病，通常会间隔很长时间。
This has been one of the hardest questions to answer because the time lag between exposure to pollutants and developing an illness is usually long.
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