单项选择题

Although the American civilization took over and replaced the frontier over a century ago, the heritage of the frontier is still evident in the United States today. Many people are still fascinated by the frontier because it has been particularly important in shaping American values. For many years, the frontier experience was romanticized in popular movies and television shows that featured cowboy heroes fighting Indian villains. Little attention was given to the tragic story of what really happened to Native Americans. Today, most Americans are more aware of the darker side of the settling of the continent, when thousands of Native American Indians were killed, their lands were taken, and much of their culture was destroyed.
The American frontier consists of the relatively unsettled regions of the United States, usually found in the western part of the country. Here, both land and life we’re more rugged and primitive than in the more settled eastern part. As one frontier area was settled, people began moving farther west into the next unsettled area, sweeping aside the Native Americans as they went. By settling one frontier area after another, Americans moved across an entire continent, 2700 miles wide. They came to believe that it was their destiny to control all the land, and eventually they did. The Native Americans were given small portions of land called reservations to control, but the United States government broke many promises and created much misery for the Indian nations.
Recently, Americans have come to a more balanced view of the settling of the West. However, many Americans still see many aspects of the frontier, its people and their beliefs, as good, inspiring examples of traditional American values in their original and purest form. How did the frontier movement, which lasted more than two centuries, help to shape these basic American values
To be sure, the frontier provided many inspiring examples of hard work as forests were turned into towns, and towns into large cities. The competitive race for success was rarely more colorful or adventurous than on the western frontier. Individualism, self-reliance, and equality of opportunity have perhaps been the values most closely associated with the frontier heritage of America. Throughout their history, Americans have tended to view the frontier settler as the model of the free individual. In the United States, where freedom from outside social controls has traditionally been valued, the frontier has been idealized, and it still serves as a basis for a nostalgic view of the early United States, a simpler time that was lost when the country became urbanized and more complex. The main idea of the passage is ______.