这道题目是比较容易选出来的,整体看这个句子,不难划分其句式结构,句子主干为“it has been foundthat...”,it 作形式主语,真正的主语为 that 引导的主语从句;而在这个主语从句,句子的主干“people can suddenly become sensitive to it”, 而 “insensitive to a certain smell”则是作为 people 的后置定语;而后面的部分为由when引导的条件状语从句相当于if/ in case引导的条件状语从句;在本句中,供选的状语修饰整个句子。由于谓语部分的频度状语“突然”(suddenly)体现了时间的交替关系,句子应该表达起初不敏感。因此,正确答案为A。而其他选项得干扰性不强,因此,这个题目的正确率也是比较高的。
However, it has been found that even people insensitive to a certain smell at first can suddenly become sensitive to it whenexposed to it often enough.