解答该题需要听懂 an incubatiop period is the time between infection and the onset of symptoms中的onset，并理解它与考题选择项中的emergence的可互换关系。 英语学习者一般十分重视词汇量的积累，这当然是必要的，但记单词绝不能只依靠 词典死记硬背，还应该在广泛阅读和听力训练中熟悉，从而真正理解所记单词的本质意义。虽然在一般词典上该题录音话语中用的onset并没有用emergence来注释， 但二者意义在这段话语中是相同的。

[重点难点]
理解在既定语境下具有同义或近义的不同词语。

[录音原文]
We're vulnerable to infectious diseases because of increased travel. Global travel enables infections to spread before symptoms develop. Infectious agents travel around in humans many times within their incubation period, an incubation period is the time between infection and the onset of symptoms, meaning people can transmit an infection though they won’t appear to be sick.