serious

他看起来是个很严肃的人。

He seems like a very ___serious person.

我的历史老师在课堂上相当严肃。

My history teacher is quite ___serious in class.

请不要笑——我是认真的。

Please don't laugh - I'm being ___serious.

他脸上带着非常严肃的表情，我知道出问题了。

He was wearing a very ___serious expression and I knew something was wrong.

她真的打算去国外生活吗？

Is she ___serious about going to live abroad?

他出了一起严重的事故。

He had a ___serious accident.