即使身处朋友之中，他也从不放松警惕。

He never ___lets ___his ___guard ___down, even when among friends.

史蒂文斯总是很谨慎，从不放松警惕。

Stevens is always cautious and never ___lets ___his ___guard ___down.

在谈判中，我一刻也不能放松警惕。

In negotiations, I can't afford to ___let ___my ___guard ___down for a moment.