阅读下面的短文，文中有15处空白，每处空白给出了4个选项，请根据短文的内容从4个选项中选择1个最佳答案。

Highways in the US The United States is well-known for its network of major highways designed to help a driver get from one place to another in the shortest possible time. (51) these wide modern roads are generally smooth and well maintained, with (52) sharp curves and many straight sections, a direct route is not always the most (53) one. Large highways often pass (54) scenic areas and interesting small towns. Furthermore, these highways generally (55) large urban centers which means that they become crowded with (56) traffic during rush hours, when the "fast, direct" way becomes a very slow route. However, there is (57) always an-other route to take if you are not in a hurry. Not far from the (58) new "superhighways", there are often older, (59) heavily traveled roads which go though the countryside. (60) of these are good two lane roads; others are uneven roads (61) through the country. These secondary routes may go up steep slopes along hilly (62) or down frightening hillsides to towns (63) in deep valleys. Though these are less direct routes, longer and slower, they generally go to places (64) the air is clean and the scenery is beautiful, and the driver may have a chance to get a fresh, clean (65) of the world.