单项选择题

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 and possible time. 1 these wide modern roads are generally 2 many well maintained, with 3 sharp curves and many straight 4 , a direct route is not always the most 5 one. Large highways often pass 6 scenic areas and interesting small towns. Furthermore, these highways generally 7 large urban centers, which means that they became crowded with 8 traffic during rush hours, 9 the "fast, direct" route becomes a very slow route. However, there is 10 always another route to take 11 you are not in a hurry. Not far from the 12 new "superhighways" there are often older 13 of heavily traveled roads which go through the countryside. 14 these are good two-lane(双重音) roads; others are uneven roads 15 through the country. These secondary routes may go up steep slopes, along high 16 , or down frightening hillsides to towns 17 in deep valleys. Through these less direct routes, longer and slower, they generally go to places 18 the air is clean and the scenery is beautiful, and the driver may have a 19 to get a fresh, clean 20 of the world.

A.Although
B.Since
C.Because
D.Therefore