Reading Comprehension 
从下列每篇短文的问题后所给的四个选择项中选出一个最佳答案。
     It is said that the United States is a nation with a mass of geography but a shortage of history, and there is some truth in it. Quite unlike the Old World, Americans' experience with their wild country was abrupt and often violent. It took less than four hundred years to deal with more than three million square miles of land. Americans occupied the greater part of the country within the last century and a half. In fact, even today much of the U.S. still remains half populated and half tamed.
     The early settlers lacked detailed geographic knowledge, but they were full of hope for the future. Unlike the blacks who were shipped to the continent as slaves, most people came of their own free will, hoping for a better life. Quite naturally they tended to think well of the new land, even when they knew little about it. More often than not, their expectations were met—and met handsomely. It is often said that American history is a tale based on geographic riches.
     Historically speaking, the U.S. was indeed geographically fortunate. Two great oceans kept it away from threats coming from Europe and Asia, sparing the country the need to maintain large military forces. Its vast territory also offered abundant environmental wealth, which allowed the nation to become self-sufficient in agriculture and minerals. In addition, magnificent system of natural waterways linked this territory together, making travel less expensive and more convenient than in other parts of the world.