Historically, the most important trade barriers have been tariffs and

Quotas.  These are the number of exports that one country can send to another.  The small the quota, the smaller the income generated.  In some cases it discourages imports to let local industries grow.  In some countries local industries have difficulty competing with foreign imports so they resort to quotas to limit their imports.

you should think about the answers you give and expect the same form your questions.