It would be "d. eight-hour days" that was not a problem faced by workers in nineteenth century mills or mines, since legislation advocating for strict working hours did not come about until the Progressive Era.
The Bessemer Process was an extremely important invention because it helped made stronger rails for constructing the railroads and helped to make stronger metal machines and innovative architectural structures like skyscrapers. The United States Industrial Revolution moved from the Age of Iron to the Age of Steel.
