Section A Multiple-Choice QuestionsPassage 1It"s one of the world"s most celebrated theories-that it takes just six steps to link any two people on the planet. Person A would have danced at a ball with B, who once shared a flat with C who bought a bicycle from D...and so on. Now a few computer whizzes have put the theory to the test and found that it is true-almost. Rather than six degrees of separation, we are linked by 6.6. In other words, we really are just a handful of acquaintances away from the likes of Madonna and the Queen. Eric Horvitz, one of the Microsoft researchers who tested the theory using electronic messages, said he was shocked at the result.The concept of six degrees of separation came to public attention after an experiment in the Sixties, but is seen today as more of an urban legend. However, the Microsoft study shows that neither the growing population-nor advances in communication technology-have markedly changed the result. "What we"re seeing suggests there may be a social connectivity constant for humanity," he said. "People have had this suspicion that we are really close. But we are showing on a very large scale that this idea goesbeyond folklore."The researchers studied the addresses of 30 billion instant messages sent through the Microsoft network in a single month in 2006. Two people were considered to be acquaintances-or separated by one degree-if they communicated with one another through the email-like system. Calculations showed the majority of users, or 78 per cent, could be connected by just 6.6 messages or steps.The phrase "six degrees of separation" came into usage after the 1960s study by academic Stanley Milgram. Milgram sent letters to a random selection of people in American cities, telling them that they were to pass the note to a certain stockbroker living in Boston if they knew him by name. If they did not, they were to pass the letter to someone they knew who they thought might have a better chance of being acquainted with him. The average number of times the letters had to be passed on to reach the broker was six, or 6.2 to be exact-and a new phrase was born.The concept was not a new one even then, and had been written about in the 1920s by Hungarian author Frigyes Karinthy. Karinthy said that any two individuals could be connected by at most five acquaintances.But after Milgram"s experiment the idea captured the world"s imagination, later spawning a play and film. In the 1993 filmSix Degrees of Separation, based on the 1990 play of the same name, one of the characters said: "Six degrees of separation between us and everybody else on the planet." "The president of the United States, a gondolier in Venice, just fill in the names." "I am bound, you are bound, to everyone on this planet by a trail of six people." However, others claim we are not as well connected as we think, and that a few quality friendships are more important that a host of loose links. Barriers such as education, class and race mean the world is not as small as we might like to believe, it is argued.Passage 2SAN FRANCISCO—After months of feverish speculation, Steven R Jobs introduced Wednesday what Apple hopes will be the coolest device on the planet: a slender tablet computer called the iPad. However, the question is whether the iPad can achieve anything close to the success of the iPhone, which transformed the cellphone and forced the industry to race to catch up.Apple is positioning the device, some versions of which will be available in March, as a pioneer in a new genre of computing, somewhere between a laptop and a smartphone. Half an inch thick and weighing 1 1/2 pounds, the device will vividly display books, newspapers, Web sites and videos on a 9.7-inch glass touch screen. Giving media companies another way to sell content, it may herald a new era for publishing.But the iPad, costing $499 to $829, also lacks some features common in laptops and phones, as technology enthusiasts were quick to point out. To its instant critics, it was little more than an oversize iPod Touch. A camera is notably absent, and Flash, the ubiquitous software that handles video and animation on the Web, does not work on the device.Another thing missing is an alternative to the AT&T data network, which is already buckling under the strain of traffic to and from iPhones. Some versions of the iPad can, for a monthly fee, use a 3G data connection like cellphones, but the only carrier mentioned was AT&T.Mr. Jobs posited that the iPad was the best device for certain kinds of computing, like browsing the Web, reading e-books and playing video. The iPad "is so much more intimate than a laptop, and it"s so much more capable than a smartphone with its gorgeous screen," he said in presenting the device to a crowd of journalists and Apple employees here. "It"sphenomenalto hold the Internet in your hands."One question Apple faces is whether there is enough room for another device in the cluttered lives of consumers. Mr. Golvin said book lovers would continue to opt for lighter, cheaper e-readers like the Amazon Kindle, while people looking for a small Web-ready computer would gravitate toward the budget laptops known as netbooks. But other analysts say they have heard similar criticism before—once aimed at the iPhone, which has now been bought by more than 42 million people around the world. These believers say Apple"s judgment on the market is nearly infallible.The success of the iPhone and its cousin, the iPod Touch, have shown a path for tablets. People have been willing to pay to customize those devices with applications, turning them into video game machines, compasses, city guides and e-book readers.Passage 3Located in tropical area at low altitudes, savannas (热带草原) are stable ecosystems, some wet and some dry consisting of vast grasslands with scattered trees or shrubs. They occur on a wide range of soil types and in extremes of climate. There is no simple or single factor that determines if a given site will be a savanna, but some factors seem to play important roles in their formation.Savannas typically experience a ratherprolongeddry season. One theory behind savanna formation is that wet forest species are unable to withstand the dry season, and thus savanna, rather than rain forest, is favored on the site. Savannas experience an annual rainfall of between 1000 and 2000 millimeters, most of it falling in a five-to-eight month wet season. Though plenty of rain may fall on a savanna during the year, for at least part of the year little does, creating the drought stress ultimately favoring grasses. Such conditions prevail throughout much of northern South America and Cuba, but many Central American savannas as well as coastal areas of Brazil and the island of Trinidad do not fit this pattern. In these areas, rainfall per month exceeds that in the above definition, so other factors must contribute to savanna formation.In many characteristics, savanna soils are similar to those of some rain forests, though more extreme. For example, savanna soils, like many rain forest soils, are typically oxisols (氧化土) (dominated by certain oxide minerals) and ultisols (老成土) (soils containing no calcium carbonate), with a high acidity notably low concentrations of such minerals as phosphorus, calcium, magnesium, and potassium, while aluminum levels are high. Some savannas occur on wet, waterlogged (溃水) soils; other dry, sandy, well-drained soils. This many seem contradictory, but it only means that extreme soil conditions, either too wet or too dry for forests, are satisfactory for savannas. More moderate conditions support moist forests.Waterlogged soils (溃水土壤) occur in areas that are flat or have poor drainage. These soils usually contain large amounts of clay and easily become water-saturated (饱水状态). Air cannot penetrate between the soil particles, making the soil oxygen-poor. By contrast, dry soils are sandy and porous, their coarse textures permitting water to drain rapidly. Sandy soils are prone to the leaching of nutrients and minerals and so tend to be nutritionally poor. Though most savannas are found on sites with poor soils (because of either moisture conditions or nutrient levels of both), poor soils can and do support lush rain forest.Most savannas probably experience mild fires frequently and major burns every two years or so. Many savanna and dry-forest plant species are called pyrophytes (耐火植物), meaning they are adapted in various ways to withstand occasional burning. Frequent fire is a factor to which rain forest species seem unable to adapt, although ancient charcoal remains from Amazon forest soils dating prior to the arrival of humans suggest that moist forests also occasionally burn. Experiments suggest that if fire did not occur in savannas in the Americas, species composition would change significantly. When burning occurs, it prevents competition among plant species from progressing to the point where some species exclude others, reducing the overall diversity of the ecosystem. But in experimental areas protected from fire, a few perennial grass species eventually come to dominate, outcompeting all others. Evidence from other studies suggests that exclusion of fire results in markedly decreased plant-species richness, often with an increase in tree density. There is generally little doubt that fire is a significant factor in maintaining savanna, certainly in most regions.In addition, humans have contributed to the conditions favoring the formation of savannas. On certain sites, particularly in South America, savanna formation seems related to frequent cutting and burning of moist forests for pastureland. Increase in pastureland and subsequent overgrazing have resulted in an expansion of savanna. The thin upper layer of humus (腐殖质) (decayed organic matter) is destroyed by cutting and burning. Humus is necessary for rapid decomposition of leaves by bacteria and fungi and for recycling by surface roots. Once the humus layer disappears, nutrients cannot be recycled and leach from the soil, converting soil from fertile to infertile and making it suitable only for savanna vegetation. Forests on white, sandy soil are most susceptible to permanent alteration.
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