单选题 New Understanding of Natural Silk's Mysteries Natural silk, as we all know, has a strength that manmade materials have long struggled to match. In a discovery that sounds more like an ancient Chinese proverb than a materials science breakthrough, MIT researchers have discovered that silk gets its strength from its weakness. Or, more specifically, its many weaknesses. Silk gets its extraordinary durability and ductility from an unusual arrangement of hydrogen bonds that are inherently very weak but that work together to create a strong, flexible" structure. Most materials—especially the ones we engineer for strength—get their toughness from brittleness. As such, natural silks like those produced by spiders have long fascinated both biologists and engineers because of their light weight, ductility and high strength (pound for pound, silk is stronger than steel and far less brittle). But on its face, it doesn't seem that silks should be as strong as they are; molecularly, they are held together by hydrogen bonds, which are far weaker than the covalent bonds found in other molecules. To get a better understanding of how silk manages to produce such strength through such weak bonds, the MIT team created a set of computer models that allowed them to observe the way silk behaves at the atomic level. They found that the arrangement of the tiny silk nanocrystals is such that the hydrogen bonds are able to work cooperatively, reinforcing one another against external forces and failing slowly when they do fail, so as not so allow a sudden fracture to spread across a silk structure. The result is natural silks that can stretch and bend while retaining a high degree of strength. But while that's all well and good for spiders, bees and the like, this understanding of silk geometry could lead to new materials that are stronger and more ductile than those we can currently manufacture. Our best and strongest materials are generally expensive and difficult to produce (requiring high temperature treatments or energy-intensive processes). By looking to silk as a model, researchers could potentially devise new manufacturing methods that rely on inexpensive materials and weak bonds to create less rigid, more forgiving materials that are nonetheless stronger than anything currently on offer. And if you thought you were going to get out of this materials science story without hearing about carbon n anotubes, think again. The MIT team is already in the lab looking into ways of synthesizing silk-like structures out of materials that are stronger than natural silk—like carbon nanotubes. Super-silks are on the horizon.

单选题It's just a petty mistake. A. major B. important C. serious D. minor

单选题How many mobile phone users will there be in Australia by the year 2000, according to the passage?

单选题It is Uobvious/U that he will win the game,

单选题Not everyone can perceive the gradual change in the writer's style.A. appreciateB. noticeC. describeD. discover

单选题Some insects rely on the tiny hairs scattered over their bodies to sense sound waves.A. amplifyB. disguiseC. sendD. detect

单选题The governor gave a rather vague outline of his tax plan.

单选题They agreed to settle the dispute by peaceful means.

单选题Many people in the region have died from the epidemic disease. A. infectious B. inferior C. infinite D. inevitable

单选题They bought the land in order to build a new office block.

单选题We all think the prices of the computers will soon plunge .

单选题Computers in Cars You"re far from home on a lonely road. Shadowy forests stretch away on both sides. A thick mist (雾) makes it difficult to see far beyond your car"s windshield (挡风玻璃). "Can this be route 90A?" you wonder. If it is, you should be near a town. Yet there"s no hint of one. Night is closing in. And you"re low on gasoline. This is a situation where an in-car computer that can navigate would be a big help. A car computer that navigates? Yes! Such computers exist. Several experimental models are being tested by General Motors, Ford, Chrysler, and several foreign car makers. These computers vary in detail. But they all contain series of maps on videodiscs or videotapes. For example, one computer system contains 13,300 maps covering the continental US. Before starting out on a trip, a driver can type in the code for the region he or she plans to drive through. The computer then shows a map of that region. At the same time, a tiny radio receiver linked to the computer goes to work. It picks up signals from navigation satellites such as the NAVSTAR network. Using these signals, the computer shows the car"s position at all times and displays this position on the map. The computer can also calculate and display the best route to follow. A navigation computer may also receive and use data about road construction, weather conditions, and traffic jams. This information would be displayed to the driver and the computer would also use the information to work out alternate (交替的) routes. Most cars nowadays also contain computers that help cars run more efficiently. Microprocessors (微处理机) control certain engine functions by regulating the mixing of fuel. Data on car speed, oil pressure, revolutions per minute, engine temperature, and fuel level can be displayed as digital data (numbers) or warning lights. Some auto designers suggest that a central computer display be used to clearly present such timely information as car speed and fuel level. Warning lights would indicate a drop in oil pressure or a sudden rise in engine temperature. To get more information on these conditions, the driver could call it up on the computer display screen. When needed, the computer could be "asked" to provide navigation aid or information about the car"s condition.

单选题Which may NOT be a reason for higher cancer death rates among US blacks?

单选题It has been said that the Acts provided a new course of action and did not merely regulate or enlarge an old one.

单选题Snow Ranger The two things—snow and mountains—which are needed for a ski area are the two things that cause avalanches, large mass of snow and ice crushing down the side of a mountain—often called "White Death." It was the threat of the avalanche and its record as a killer of man in the western mountains that created the snow ranger. He first started on avalanche control work in the winter of 1937-38 at Alta, Utah, in Wasatch National Forest. This mountain valley was becoming well known to skiers. It was dangerous. In fact, more than 120 persons had lost their lives in 1936 and another 200 died in 1937 as a result of avalanches before it became a major ski area. Thus, development of Alta and other major ski resorts in the west was dependent upon controlling the avalanche. The Forest Service set out to do it, and did, with its corps of snow rangers. It takes many things to make a snow ranger. The snow ranger must be in excellent physical condition. He must be a good skier and a skilled mountain climber. He should have at least a high school education, and the more college courses in geology, physics, and related fields he has, the better. He studies snow, terrain, wind, and weather. He learns the conditions that produce avalanches. He learns to forecast avalanches and to bring them roaring on down the mountainsides to reduce their killing strength. The snow ranger learns to do this by using artillery, by blasting with TNT, and by the difficult and skillful art of skiing avalanches down. The snow ranger, dressed in a green parka which has a bright yellow shoulder patch, means safety for people on ski slopes. He pulls the trigger on a 75 mm. Recoilless rifle, skis waist deep in powder testing snow stability, or talks with the ski area's operator as he goes about his work to protect the public from the hazards of deep snow on steep mountain slopes.

单选题The price of vegetables {{U}}fluctuates{{/U}} according to the weather.

单选题His motive for working so hard is that he needs money.A. motivationB. activeC. motionD. nature

单选题Animal"s "Sixth Sense" A tsunami was triggered by an earthquake in the Indian Ocean in December, 2004. It killed tens of thousands of people in Asia and East Africa. Wild animals, 1 , seem to have escaped that terrible tsunami. This phenomenon adds weight to notions that they possess a "sixth sense" for 2 , experts said. Sri Lankan wildlife officials have said the giant waves that killed over 24,000 people along the Indian Ocean island"s coast clearly 3 wild beasts, with no dead animals found. "No elephants are dead, not 4 a dead rabbit. I think animals can 5 disaster. They have a sixth sense. They know when firings are happening," H. D. Ratnayake, deputy director of Sri Lanka"s Wildlife Department, said about one month after the tsunami attack. The 6 washed floodwaters up to 2 miles inland at Yala National Park in the ravaged southeast, Sri Lanka"s biggest wildlife 7 and home to hundreds of wild elephants and several leopards. "There has been a lot of apparent evidence about dogs barking or birds migrating 8 volcanic eruptions or earthquakes. But it has not been proven, "said Matthew van Lierop, an animal behavior 9 at Johannesburg Zoo. "There have been no 10 studies because you can"t really test it in a lab or field setting, "he told Reuters. Other authorities concurred with this 11 . "Wildlife seem to be able to pick up certain 12 , especially birds...there are many reports of birds detecting impending disasters," said Clive Walker, who has written several books on African wildlife. Animals certainly 13 on the known senses such as smell or hearing to avoid danger such as predators. The notion of an animal "sixth sense"—or 14 other mythical power—is an enduring one which the evidence on Sri Lanka"s ravaged coast is likely to add to. The Romans saw owls 15 omens of impending disaster and many ancient cultures viewed elephants as sacred animals endowed with special powers or attributes.

单选题Ordinary plants are unable to survive in the desert mainly because of the changeable weather.

单选题We have never seen such gorgeous hills.