I believe it is A. religious wars and conflicts related to the beliefs of the two largest faiths.
He desegregated the military with an executive order
The RMS Lusitania was a ship that sailed from New York and had American civilians in - at that point the US was neutral in the WWI -, when the ship came to the sight of the Irish Coast the German U-boat attacked it.
Despite the fact that Americans were horrified at the Lusitania attack, many Americans did not like the idea to enter the war, Irish Americans did not enjoy the idea of helping Britain because the country denied Ireland its independence, German Americans were sympathetic towards their ancestral homeland and Americans saw no reason to enter into a war between European powers. The US only entered war two and a half years after it started.
I believe the answer is Barbed Wire.
Befor the inventions of barbed wire, the Cattles are let to roam freely across the plains and the owners are required to pay a lot of cowboys to supervise them so they didn't go missing. Witht he invention of barbed wire, they can keep the cattle in a secure place without supervision.
