He received the letter challenging him to a duel and began preparing for the duel.

He is a carnivorous man, being aggressive in relationships.

Gender roles are an important element in defining individual behavior in society.

His role is to fulfill his responsibilities as a leader.

It is important to understand the importance of health.

He is an expert in ecology.

He is very interested in ecology.

The Solar System is composed of eight planets, including Earth.

In this experiment, we will use the International System of Units.

We use this coordinate system to locate the position of an object.