The justification that Southern states used in the 1800s to nullify tariff laws were states' rights
Answer:
By them moving to the frontier, they were advertising how they wanted to move there. They would show people the amazing aspects of the rest of the country, inspiring them to move there as well.
The right answer is the third one: the bubonic plague originated in the Gobi Desert, part of the then Mongol Empire, around 1320. It then propagated to China and subsequently to India, Europe and Russia. The disease spread via trade routes mainly as a result of a poor hygiene. It never reached the American continent or Sub-Saharan Africa.
