单选题
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.【C1】______these wide modern roads are generally【C2】______and well maintained. With【C3】______sharp curves(弯道)and many【C4】______sections, a direct route is not always【C5】______enjoyable one. Large highways often pass【C6】______scenic areas and interesting small towns. Furthermore, these highways generally【C7】______large urban centers, which means that they become crowded with【C8】______traffic during rush hours, when the "fast, direct" way becomes a very【C9】______route. However, there【C10】______almost always another route to take【C11】______you are not in a hurry. Not far【C12】______the relatively new "superhighways" , there are often older,【C13】______heavily traveled roads which go through the countryside.【C14】______of these are good two-lane(双车道)roads; others are uneven roads curving through the country. These secondary routes may【C15】______steep slopes(陡坡), along high cliffs, or down frightening hillsides to towns【C16】______in deep valleys. Through these less direct【C17】______, longer and slower, they generally go to places【C18】______the air is clean and the scenery is beautiful, and the driver may have a【C19】______to get a fresh, clean【C20】______of the world.