What is the explanation for his behavior?

He served as a stand-in when the lead actor in the movie fell ill suddenly.

He has no knowledge about anal sex.

His explanation was very concise and easy to understand.

His speech was concise and easy to understand.

This story has finally reached its conclusion.

This story finally came to an end.

There is a theory that there are geysers on Mars.

He asked me three questions.

He threw a difficult question at me.