The correct answer is: "The migration from rural to uban areas".
Before industrialization, the largest proportion of the world population lived in rural areas and worked in agricultural activities for subsistence, using very rudimentary tools and technological developments, that permitted very few production surpluses and trade activites.
When industrialization started, it brought a total efficiency revolution to the production model. Mass-production in factories, located in urban areas, created many job positions, where higher wages were offered and therefore, many people were attracted from the countryside to these newly created employment environments.
It did help lower shipping costs, and it did bring prosperity to towns, so it's either A or C, and I'm pretty sure it helped unite the growing country.
