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3. However, it has been found that even people insensitive to a certain smell at first can suddenly become sensitive to it when exposed to it often enough.
【分析】多重复合句。句子主干为 it has been found that...，it 作形式主语，真正的主语为 that 引导的主语从句。该从句的主干为 people...can suddenly become sensitive...。其中 insensitive to a certain smell at first 是 people 的后置定语， when 引导的条件状语从句省略了主语和系动词。
【译文】然而我们却发现，即使某些人开始时对某种气味不敏感，如果经常接触这种气味，也会对它敏感起来。
【点拨】1) be insensitive to sth.“对...不敏感的”。2) be exposed to sth.“暴露于；接触”。
【临摹】这种液体不可暴露在空气中，因为它会很快蒸发掉。
4. The explanation for insensitivity to smell seems to be that the brain finds it inefficient to keep all smell receptors working all the time but can create new receptors if necessary.
【分析】多重复合句。句子主干为 The explanation...seems to be that...。 that 引导表语从句，该从句中由 but 连接的两个并列句组成；第一个分句中 it 作形式宾语，其后的不定式结构 to keep...作真正的宾语；第二个分句中 if necessary 为条件状语从句的省略结构， but 前省略相同的主语 the brain。
【译文】对气味不敏感的解释似乎是：大脑发现让所有气味接收器一直工作效率太低，但是在需要的时候仍可以产生新的感受器。
