Article IV promises the federal government will protect each state against invasion. Hope this helps you!
The answer to your question is true :)
Easy. By learning and studying language patterns.
It's mainly because of A and B, because after the war, the industrial resolution happened, and farmers are replaced by machines, farmers has no work to do, their speed is no compare to the machines, so they must move to another place. Also the population in Europe is growing in a extremely fast speed. Hope that helps :)