The correct answer is - citizenship.
The citizenship represents a legal relationship between a particular country and an individual. Usually the country in question is a country where the individual has been born, lives, works, or got married, and in turn the individual get protection from that country and is able to live there.
Take in notice that not all countries have citizenship as an official document, some just do it by using ID cards, or passports, while some countries do have a special official document that confirms the citizenship.
I think the answer is false because a lot of North Koreans escape China by walking. Plus, they don't have much money to buy a boat anyways.
They wanted to compete with other european countries and get more wood and resources and gold
It is technically false that the Frontier ended when the open land was gone, since the open land was never "gone" just either taken up by private owners.
