单项选择题

The United States is well-known for its 【B1】 of major higher highways designed to help a 【B2】 get from one place to another in the shortest possible time. 【B3】 these wide modem roads are generally 【B4】 and well maintained, with much sharp curves and many straight 【B5】 ,a direct route is not always the most 【B6】 one. Large 【B7】 of- ten pass by scenic areas and interesting small towns. Furthermore, these highways generally 【B8】 large urban centers, which means that they become crowded with 【B9】 traffic during rush hours, 【B10】 the" fast, direct" route becomes a very slow route. However, there is still al- ways another route to take 【B11】 you are not in a hurry. Not far from the 【B12】 new" superhighways", there are often older, 【B13】 ______heavily traveled roads which go through the countryside. All of these are good two-lane roads; others are uneven roads 【B14】 through the country. These secondary routes may 【B15】 ______steep slopes, along high 【B16】 , or down frightening hillsides to towns 【B17】 ______in deep valleys. Through these less direct routes, longer and slower, they generally go to places 【B18】 the air is clean and the scenery is beautiful, and the driver may have a 【B19】 to get a fresh, clean 【B50】 of the world.