The United States is well-known for its network ___（1）___ major highways designed to help a driver get from one place to another in the ___（2）___ possible time. However, large highways often pass ___（3）___ scenic areas and interesting small towns. Furthermore, these highways generally connect large urban centers, ___（4）___ means that they become crowded with heavy traffic during rush hours, ___（5）___ the "fast direct" way becomes a very slow route.
However, there is always another route to take ___（6）___ you are not in a hurry. Not far from the relatively new "superhighways", there are often older, less heavily traveled roads which go through the countryside. ___（7）___of these are good two-lane roads; others are uneven roads curving through the country. These secondary routes may go up steep slopes, along high ___（8）___, or down frightening hillsides to towns ___（9）___ in deep valleys. Through these less direct routes, longer and slower, they generally go to places where the air is clean and the scenery is beautiful, and the driver may have a chance to get a fresh, clean ___（10）___ of the world.
（6）下列正确的是（ ）

A.unless

B.if

C.as

D.since