他是认真的，不是在开玩笑。

He is ___serious, not joking.

环境污染是一个需要立即解决的严重问题。

Environmental pollution is a ___serious issue that needs immediate attention.

这是一个需要认真对待的问题。

This is a ___serious matter that requires careful attention.

这次会议讨论了一个严重问题。

The meeting discussed a ___serious issue.