他脸上的严肃表情让我意识到问题的严重性。

The ___seriousness on his face made me realize the gravity of the situation.

我们必须认识到这个问题的严重性，并立即采取行动。

We must recognize the ___seriousness of this issue and take immediate action.

警察向我们强调了这次事故的严重性。

The police stressed the ___seriousness of the accident to us.