I am majoring in psychology.

Geography is my favorite subject.

He is an expert in volcanology, studying volcanoes around the world.

He is a world-renowned agronomist.

What is the difference in price between these two products?

He is an expert in biochemistry.

The use of chemical weapons is prohibited by international law.

He is known as a person of erudition.

He is majoring in climatology at university.

He is an expert in limnology, and his research is highly regarded worldwide.