【正确答案】 D

【答案解析】[听力原文]
   Nowadays motorcycling is fast becoming one of Americans' most popular sports. However, there are several things about motorcycling that the average citizen dislikes. Motorcyclists frequently get dirty. In fact, they are dirty. On the road there is little to protect them from mud, insects and bird droppings. For practical reasons, they are often dressed in old clothing, which looks much less respectable than the clothing of people who ride in cars. For the same reason, motorcyclists usually wear dark colors. Of course, the danger of motorcycling also helps account for many people's low opinion of the sport. Its defenders, however, claim that careful cyclists are in less danger than it's commonly believed. He must pay careful attention to his driving. From that point of view, a man on a motorcycle is safer than a man in a car.
   Why do motorcyclists often dress in old and dark clothing?