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。

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