In the mid-nineteenth century, the United States had tremendous natural resources that could beexploited in order to develop heavy industry. Most of the raw materials that are valuable in themanufacture of machinery, transportation facilities, and consumer goods lay ready to be worked intowealth. Iron, coal, and oil-the basic "INGREDIENTS" of industrial growth-were plentiful andneeded only the application of technical expertise, organizational skill, and labor.One crucial development in this movement toward industrialization was the growth of therailroads. The railway network expanded rapidly until the railroad map of the United States lookedlike a spider's web, with the steel filaments connecting all important sources of raw materials, theirplaces of manufacture, and their centers of distribution. The railroads contributed to the industrialgrowth not only by connecting these major centers, but also by "THEMSELVES" consumingenormous amounts of fuel, iron, and coal.Many factors influenced emerging modes of production. For example, machine tools, the toolsused to make goods, were steadily improved in the latter part of the nineteenth century-always with aneye to speedier production and lower unit costs. The products of the factories were rapidly absorbedby the growing cities that sheltered the workers and the distributors. The increased urban populationwas "NOURISHED" by the increased farm production that, in turn, was made more productive bythe use of the new farm machinery. American agricultural production kept up with the urban demandand still had surpluses for sale to the industrial centers of Europe.The labor that "RAN" the factories and built the railways was recruited in part from Americanfarm areas where people were being displaced by farm machinery, in part from Asia, and in part fromEurope. Europe now began to send tides of immigrants from eastern and southern Europe-most ofwhom were originally poor farmers but who settled in American industrial cities. The money tofinance this tremendous expansion of the American economy still came from European financiers forthe most part, but the Americans were approaching the day when their expansion could be financed intheir own "money market."The bold and uppercase word "THEMSELVES" refers to ..a. sourcesb. centersc. railroadsd. places