so it is often difficult to determine if the symptoms are a result of　2　to indoor air pollution.

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【解析】选[C]。空前的0f表明此处应填一名词，且该名词应能与to配使用。首句中出现的experienced soon after exposure对此有提示作用。此处是说，室内空气污染的某种直接影响与感冒和病毒性疾病的症状很类似，所以通常很难确定是不是室内空气污染引起的症状，[C]exposure(暴露)符合语义。

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1 it is important to pay attention to the time and place　3　occur.

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2 Other health effects may show up either years after exposure has occurred or only after long or　4　periods of exposure.

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3 These effects, which　5　some breathing diseases, heart disease

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so it is often difficult to determine if the symptoms are a result of 2 to indoor air pollution.

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so it is often difficult to determine if the symptoms are a result of　2　to indoor air pollution.