单选题
However,it has been found that even people insensitive to a certain smell at first can suddenly become sensitive to it when（ ）to it often enough.
Asubjected
Bleft
Cdrawn
Dexposed
正确答案
答案解析
本题所在的句子是一个由when引导的条件句，由主句的主语可以推断，该从句省略了主语和谓语即people are。同时，从句中的it指代的是上文的a certain smell，由此可推断本句意思是：如果人能经常闻到这种气味也能突然对它变得敏感。而四个选项中，只有D.expose一词能与to搭配表示“使某人接触到……”，在这里引申为闻到气味。