D.
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Railroads played an absolutely crucial role in the Industrial Revolution, since they made for the incredibly fast transfer of goods such as iron and steel that were used to build things--not to mention the transport of the workers themselves.
Hunter-Gatherers were nomadic tribes who relied on moving to different locations in search of fertile hunting/gathering grounds. Sometimes, these lands they explored were barren, while some were plentiful, but the key was to keep moving, so they could not stay in the fertile land for long.
Eventually, humans started to farm, and domesticate animals, which led into the Neolithic Era (civilization). Farming gave humans the assurance of constant food, and furthermore, set more social roles and behavioral standards for communities. However, this transition caused the loss of equality for different people in a community, and raised the style of living we know, with the establishment of classes and governments.