What are the main physical trade barriers of Europe?

I have this huge upcoming test in Social Studies and our theme in that class for this 9 weeks was Europe. But anyway, he gave us a study guide and one of the statements says ‘Must know the physical trade barriers of Europe’. I don’t know if he means for us to say mountains, grasslands, and rivers, or does he mean specific such as Pyrenees, European Plain, etc. But either way please give me the answers so I can put them on the study guide and study for the 50% of my grade test which is tomorrow! Thanks