First it was jogging. Then aerobics（有氧运动）. Not too long ago, Americans discovered race walking.
Now Americans are into a new fitness craze. They’re taking up bicycling. Over hills and down mountainsides and across quiet country roads, Americans are busily rolling along.
The number of adults who ride for fitness is around 17 million, an increase of 70 percent over four years ago. Twice as many women as men are coming to the sport. Americans are falling in love with biking because it has speed, the benefits of jogging and beautiful scenery.
Bicycling is a very appropriate sport, which is important to people who injured their knees while jogging or whose joints are aching from aerobics. And biking is a real awakening for people who have been into race walking in the past. Race walking is as dull as watching paint dry.
The most popular kind of bicycle for people who are new to the sport is the mountain bike, which has a fixed frame with wide tires and upright handles. Mountain bikes also have many gears（齿轮） to make it easier to climb hills. About 5 million Americans ride mountain bikes, compared with 200,000 who rode them only five years ago.
Costs range from about ＄130 for a bottom-of-the-line bicycle to more than ＄2,700 for an expensive bicycle.
Mountain biking has attracted some people who race down the sides of mountains like a bat out of hell. But most riders ride slowly and they rarely venture far from home.
The biking craze has brought an unexpected profit（盈利） to clothing and bicycle accessory（附属品）makers. Last year, bikers paid ＄630 million for biking clothes and accessories.
Bicycling seems likely to continue its fantastic growth.
53. Race walking is about as dull as watching paint dry because _______         
A. race walking is a slow-moving sport
B. the number of adults who ride for fitness has grown 70 percent in four years.
C. it has speed, the benefits of jogging and beautiful scenery.
D. Americans are taking up bicycling.
54. The bicycling craze has been a profit for _______.
A. people who want to ride like a bat out of hell
B. bicycle accessory makers
C. race walkers
D. twice as many women as men
55. What does the underlined word “bottom-of-the-line” mean?
A. poor       B. modern    C. old        D. cheapest
56. The main idea of the article is _______
A. riding a bicycle is one of the most dangerous sports in America
B. Americans are rolling along
C. bicycling is the latest fitness craze to hit America
D. most people in America want to own a hand-made bicycle that can cost more than 2,700