What is the thesis (main point) of diamond's article?

Which of the following was probably the most difficult challenge facing construction of the railroad?

In the latter stages of the civil war who was given the responsibility of winning the war for the north

The service industry in the united states grew in the late 1800s. what was one effect of that growth? the number of people with jobs in factories as unskilled workers decreased. the number of people with jobs in factories as unskilled workers increased. the middle class grew as people took jobs as clerks, lawyers, and teachers. the middle class shrank as people left jobs as clerks, lawyers, and teachers.