公司被警告要对可疑包裹保持警惕。

Companies were warned to ___be ___on ___their ___guard for suspicious packages.

然而，我们应该保持警惕，以防情况升级。

However, we should ___be ___on ___guard to prevent situations from escalating.

在不确定时期，投资者需要警惕市场波动。

In uncertain times, investors need to ___be ___on ___their ___guard against market volatility.