[originaltext] The territorial growth in the United States was followed by gr

The territorial growth in the United States was followed by great industrial expansion. The country was rich in natural resources. A constant flow of immigration provided the labor force necessary to develop these resources. New industries were started. Chicago, St. Louis, and Detroit grew from small towns to large cities. The United States became the greatest industrial country in the world. (4) Detroit’s growth has been typical of these new American cities. Situated on the Detroit River close to the Great Lakes, it had the geographical advantages necessary for industrial growth. (6) Factories for manufacture of bicycles and wagon bodies were established there. With the coming of the automobile, these factories were converted into automobile plants. The American system of mass production was introduced. The industry grew and Detroit grew with it. (4)Detroit today is the fifth largest city in the United States. Four Detroit automobile companies produce 96 percent of all American cars. Twenty thousand of these cars roll off Detroit assembly lines each day. (5) Some of the factories there never stop. They have three groups or “shifts” of workers. Each shift works eight hours and is relieved by a new shift, making all together a twenty-four-hour day. So Detroit has been called “The City of Three Shifts”. As a result, in the Ford plant it is possible to build an entire car in less than twenty-eight hours. In little more than a day, pure iron can be converted into a shiny new car of the latest model.
4. What is the main topic of the talk?
5. How do some of the factories there never stop?
6. What did the factories manufacture before being converted into automobile plants?