Why did Alexander's empire stop when and where it did? A
He had conquered all of the lands that he wanted.

B
He could not cross the mountains of Asia.

C
His soldiers were war-weary and wanted to go home.

A  trade barrier is when a government forces restriction on exchange of goods and services, while a migration barrier prevents and individual from moving from one country to another.

Explanation:

A trade barrier or barriers is refereed to as when a government forces a restriction on the  international free exchange of services or goods.

Trade barriers are categorized as, customs practices, quotas, import licensing, labeling, standards, testing, import policies reflected in tariffs and other import charges, and various types of certification.

Migration barriers can be defined as when a person or prevented or not allowed to move from one  to another country.

National Barriers comprises of  both physical and political factors that have major obstacles to migration.