https://online2.tingclass.net/lesson/shi0529/0009/9907/926.mp3
https://image.tingclass.net/statics/js/2012
讲解文本：
lower one's guard 丧失警惕，疏忽大意，轻视而漫不经心
We should never lower our guard at any time.
任何时候我们都不应该掉以轻心。
It's not safe here, so you can never lower your guard.
这里不安全，所以千万别掉以轻心。
