https://online2.tingclass.net/lesson/shi0529/0009/9907/926.mp3
https://image.tingclass.net/statics/js/2012
講解文本：
lower one's guard 喪失警惕，疏忽大意，輕視而漫不經(jīng)心
We should never lower our guard at any time.
任何時候我們都不應(yīng)該掉以輕心。
It's not safe here, so you can never lower your guard.
這里不安全，所以千萬別掉以輕心。
