there is considerable　8　about what concentrations or periods of exposu-考呗网

英语四级

填空题there is considerable　8　about what concentrations or periods of exposure are necessary to produce specific health problems.

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【解析】选[A]。空前的形容词considerable表明此处应填一名词。此句是说，对于暴露在空气污染中时间长短是否能够影响身体健康程度这个问题还不太确定，[A]uncertainty(不确定)符合语义。

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