题目内容
In the speed of its execution, the 【C5】______ of a tumbling cat resembles a magician's trick. The gyrations of the cat in midair are 【C6】______ fast for the human eye to follow, so the process is obscured.【C7】______ the eye must be speeded up, or the cat's fall slowed down for the phenomenon to be observed. A century ago the former was accomplished 【C8】______ high-speed photography using equipment now 【C9】______ in any pharmacy. But in the nineteenth century the 【C10】______ on film of a falling cat constituted a scientific experiment.
The experiment was described in a paper presented to the Paris Academy in 1894. Two sequences of twenty photographs【C11】______ one from the side and one from behind, 【C12】______ a white cat in the act of righting itself. Grainy and quaint 【C13】______ they are, the photos show that the cat was dropped upside down, with no initial spin, and still landed on its feet. Careful analysis of the photos reveals the secret. As the Cat rotates the front of its body 【C14】______ , the rear and tail twist counterclockwise, 【C15】______ the total spin remains zero, 【C16】______ Newton's laws. Halfway down, the cat pulls in its legs before reversing its twist and then extends them again, 【C17】______ the desired end result. The explanation was that while no body can acquire spin without torque, a flexible one can readily change its 【C18】______ . Cats know this instinctively, but scientists could not be sure how it happened 【C19】______ they increased the speed of their 【C20】______ a thousandfold.
【C1】
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How Sled Dogs Work
They're physical wonders-able to endure extreme conditions for extensive periods of time while running fast and pulling more than their own weight. Sled (雪橇) dogs have been around for generations, and today they remain a large part of a dog driver's life.
In Arctic (北极的) regions, some 35 000 years ago, Eskimos' dogs bred with wolves and other dogs. Their offspring were well-adapted to the cold and snow. There's no saying exactly when someone thought to have dogs pull sleds, but it was well before 1000 B. C. that dog sleds provided a means of transportation and a way to haul goods. In recent years, people looking for unique outings have gone on recreational trips with sled dogs. A number of companies market tourist treks via dog sled for adventure travelers.
Dogsled racing in Alaska formally began in 1908, when sled dog owners in Nome, Alaska, held the first All-Alaska Sweepstakes, a 408-mile race from Nome to Candle and back. And in 1973, the inaugural Iditarod Trail Sled Dog Race took place on a route covering more than 1 000 miles in Alaska. There are more competitions in many of the continental United States, and some take place on dry land. In addition to endurance races, sled-dog events include sprint races, weight pulls (in which dogs try to pull the most weight) and freight races (in which dogs try to pull a weighted sled the fastest).
Stamina (耐力) and Strength
Many breeds of dogs are used to pull sleds, including official American Kennel Club breeds such as the Siberian husky, Malamute and Samoyed. However, many sled dogs are of mixed breed and are called Alaska husky or Eskimo dogs. While some Sled dogs may have wolf bloodlines, it's generally not desirable to have a mixture of blood between the two.
Sled clogs are bred for their speed and endurance as well as leadership qualities. They are big dogs with thick coats and wide, flat feet. They sleep with their tails covering their noses to keep warm. Dogs that weigh around 40 to 45 pounds are the ideal size to pull sleds, but some may weigh as much as 85 pounds.
Young dogs that are enthusiastic runners, easy-going and mentally tough are good choices for sled dogs. A dog's gender matters little and both male and female dogs are considered equally.
For racing, the most intelligent and fastest dogs are picked to be lead dogs and run in the front of the pack. Behind them run swing dogs, whose job is to direct the team around turns and curves. At the back of the dog team are the wheel dogs or wheelers, who are right in front of the sled and are usually the largest and strongest of the team. The rest of the dogs are known simply as team dogs.
Dogs are typically from 2 to 10 years old when they pull sleds. After they retire, they might remain with their owner at a kennel or be adopted as pets by others. Rescue groups for sled dogs take in unwanted dogs and care for them, trying to place them in loving homes.
While snowmobiles (also called snow machines),helicopters and airplanes are now often used !n Place of sled dogs, there are still times when dog transportation is preferred. Harsh weather conditions can make flying hazardous, and certain terrain can be more difficult for snow machines to cross than for dogs. Dogs can be more reliable than man-made machines, they're good companions to have on long treks and they can be cheaper to own than other forms of transportation.
Becoming a Sled Dog
The number of sled dogs at a typical kennel(养狗场)is around 75, though some might house as few as 20 Or as many as 150. Many kennel owners are greedy mushers (赶狗拉雪橇的人) and compete in dog sled races. They also train other mushers and sell and lease dogs. The training for each team of racing dogs varies by musher (or whomever is preparing the dogs to race). Conditioning the dogs to run long d
M: I'd love to, but I've got a history final in two days, and I haven't studied all quarter. I think I'd better take a rain check this time.
Q: What does the man mean？
(19)
A．His job as a police officer.B．His personal life.C．How stressful patrol work is.D．How
The supply of wine has remained the same partly because ______.
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