In the early 1800's, over 80 percent of the United States labor force was engaged in agriculture. Sophisticated technology and machinery were virtually nonexistent. People who lived in the cities and were not directly involved in trade often participated in small cottage industries making handcrafted goods. Others cured meats, silversmiths, candle or otherwise produced needed goods and commodities. Blacksmiths, silversmiths, candle makers, and other artisans (手艺人) worked in their homes or barns, relying on help of family.
Perhaps no single phenomenon brought more widespread and lasting change to the United States society than the rise of industrialization. Industrial growth hinged on several economic factors. First, industry requires an abundance of natural resources, especially coal, iron ore, water, petroleum, and timber--all readily available on the North American continent. Second, factories demand a large labor supply. Between the 1870's and the First World War (1914-1918), approximately 23 million immigrants streamed to the United States, settled in cities, and went to work in factories and mines. They also helped build the vast network of canals and railroads that crisscrossed the continent and linked important trade centers essential to industrial growth.
Factories also offered a reprieve (暂时减轻) from the backbreaking work and financial unpredictability associated with farming. Many adults, poor and disillusioned (大失所望) with farm life, were lured to the cities by promises of steady employment, regular paychecks, increased access to goods and services, and expanded social opportunities. Others were pushed there when new technologies made their labor cheap or expendable; inventions such as steel plows and mechanized harvesters allowed one farmhand to perform. work that previously had required several, thus making farming capital-intensive rather than labor-intensive.
The United States economy underwent a massive transition and the nature of work was permanently altered. Whereas cottage industries relied on a few highly skilled craft workers who slowly and carefully converted raw materials into finished products from start to finish, factories relied on specialization. While factory work was less creative and more monotonous (单调无聊的), it was also more efficient and allowed mass production of goods at less expense.
What aspect of life in the United States does the passage mainly discuss？

A. The transition from an agricultural to an industrial economy.
B. The inventions that transformed life in the nineteenth century.
C. The problems associated with the earliest factories.
D. The difficulty of farm life in the nineteenth century,