题目内容：
Highways in the USThe 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 another 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.回答第51空。 A．A Although
B．B Since
C．C Because
D．D Therefore
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