The reason why there was an increase in the total US urban population during the early 20th century is becasue of job opportunities in industry and improved living conditiosn in cities. But the most likely reason would probably be A - job opportunities in industry.
During the Colombian trade people traded slaves, corn, diseases, horses, tobacco, cotton, and more.
D is correct c might be correct but not sure try it at your own risk.
In June 1914 a Serbian-nationalist terrorist group called the Black Hand sent groups to assassinate the Archduke