America's foundation rests on its agriculture industry. This industry feeds all of the citizens of the country. It also exports food to other countries around the world. However, this industry did not develop instantly. It took centuries of refinement to create an agricultural base that is capable of sustaining the entire population.The westward movement of colonies spread farms across the U.S. Settlers traveled to new areas and built their farms. Their efforts created new towns and cities. The supply chains that led to these new areas became the roads that run throughout the nation.Wheat was often the crop of choice in northern regions. The reason for this is that wheat crops are easily grown in cool regions. This crop was usually planted in newly settled lands. These new areas were known as the "wheat frontier." This zone of newly settled wheat farms moved west over the years. After these wheat farms moved on, more diversified farms took their place.In the Midwest, the farming of corn and raising of hogs was a common agricultural combination. Hogs and corn complement each other. This is because in the time before canals and railroads, it was difficult to get grain to market. So the grain could be fed to the hogs, which were much easier to transport.In the warm southern regions, cotton and herds of beef cattle were the most popular products. The reason behind this was that both flourish in the heat. Tobacco farming was also common in the South. Until the Civil War, this was done through the use of slave labor. Slaves were also used in agriculture in the Northeast. They were used there up to the early 1800s. But slavery was prohibited in the Midwest. The Freedom Ordinance of 1787 outlawed it.During the time of the Great Depression, huge areas of the Midwest were abandoned. This was due to the Dust Bowl storms that swept through the region and rendered the soil useless for farming. These regions were then made into national forests. But in the 1940s, efforts to use these areas for farmland resumed in support of the World War II effort.Which of the following would most appropriately follow the last paragraph of the passage...