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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】多重復合句。句子主干為 it has been found that...，it 作形式主語，真正的主語為 that 引導的主語從句。該從句的主干為 people...can suddenly become sensitive...。其中 insensitive to a certain smell at first 是 people 的后置定語， when 引導的條件狀語從句省略了主語和系動詞。
【譯文】然而我們卻發現，即使某些人開始時對某種氣味不敏感，如果經常接觸這種氣味，也會對它敏感起來。
【點撥】1) be insensitive to sth.“對...不敏感的”。2) be exposed to sth.“暴露于；接觸”。
【臨摹】這種液體不可暴露在空氣中，因為它會很快蒸發掉。
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】多重復合句。句子主干為 The explanation...seems to be that...。 that 引導表語從句，該從句中由 but 連接的兩個并列句組成；第一個分句中 it 作形式賓語，其后的不定式結構 to keep...作真正的賓語；第二個分句中 if necessary 為條件狀語從句的省略結構， but 前省略相同的主語 the brain。
【譯文】對氣味不敏感的解釋似乎是：大腦發現讓所有氣味接收器一直工作效率太低，但是在需要的時候仍可以產生新的感受器。
