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For centuries, explorers have risked their lives venturing into the unknown for reasons that were to varying degrees economic and nationalistic. Columbus went west to look for better trade routes to the Orient and to promote the greater glory of Spain, Lewis and Clark journeyed into the American wilderness to find out what the U. S. had acquired when it purchased Louisianan, and the Apollo astronauts rocketed to the moon in a dramatic show of technological muscle during the cold war.Although their missions blended commercial and political-military imperatives, the explorers involved all accomplished some significant science simply by going where no scientists had gone before.Today Mars looms as humanity"s next great terra incognita (未探明之地). And with doubtful prospects for a short-term financial return, with the cold war a rapidly fading memory and amid a growing emphasis on international cooperation in large space ventures, it is clear that imperatives other than profits or nationalism will have to compel human beings to leave their tracks on the planet"s reddish surface. Could it be that science, which has long played a minor role in exploration, is at last destined to take a leading role The question naturally invites a couple of others: are there experiment that only human could do on Mars Could those experiments provide insights profound enough to justify the expense of sending people across interplanetary spaceWith Mars the scientific stakes are arguably higher than they have ever been. The issue of whether life ever existed on the planet, and whether it persists to this day, has been highlighted by mounting evidence that the Red Planet once had abundant stable, liquid water and by the continuing controversy over suggestions that bacterial fossils rode to Earth on a meteorite (陨石) from Mars. A more conclusive answer about life on Mars, past or present, would give researchers invaluable data about the range of conditions under which a planet can generate the complex chemistry that leads to life. If it could be established that life arose independently on Mars and Earth, the finding would provide the first concrete clues in one of the deepest mysteries in all of science, the prevalence of life in the universe. At present, a probable inducement for countries to initiate large-scale space ventures is ______.
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Every year television stations receive hundreds of complaints about the loudness of advertisements. However, federal rules forbid the practice of making ads louder than the programming. In addition, television stations always operate at the highest sound level allowed for reasons of efficiency. According to one NBC executive, no difference exists in the peak sound level of ads and programming. Given this information why do commercials sound so loudThe sensation of sound involves a variety of factors in addition to its speak level. Advertisers are skilful at creating the impression of loudness through their expert use of such factors. One major contributor to the perceived loudness of commercials is that much less variation in sound level occurs during a commercial. In regular programming the intensity of sound varies over a large range. However, sound levels in commercials tend to stay at or near peak levels.Other "tricks of the trade" are also used. Because low-frequency sounds can mask higher frequency sounds, advertisersfilter outany noises that may drown out the primary message. In addition, the human voice has more auditory (听觉的) impact in the middle frequency ranges. Advertisers electronically vary voice sounds so that they stay within such a frequency band. Another approach is to write the script so that lots of consonants are used, because people are more aware of consonants than vowel sounds. Finally, advertisers try to begin commercials with sounds that are highly different from those of the programming within which the commercial is buried. Because people become adapted to the type of sounds coming from programming, a dramatic change in sound quality draws viewer"s attention. For example, notice how many commercials begin with a cheerful song of some type.The attention-getting property of commercials can be seen by observing one-to two-year-old children who happen to be playing around a television set. They may totally ignore the programming. However, when a commercial comes on, their attention is immediately drawn to it because of its dramatic sound quality. According to the passage, the maximum intensity of sound coming from commercials ______.
Most people think of lions as strictly African beasts, but only because they"re been killed off almost everywhere else. Ten thousand years ago lions spanned vast sections of the globe. Now lions hold only a small fraction of their former habitat, and Asiatic lions, a subspecies that split from African lions perhaps 100,000 years ago, hang on to an almost impossibly small slice of their former territory.India is the proud steward of these 300 or so lions, which live primarily in a 560-square-mile sanctuary (保护区). It took me a year and a half to get a permit to explore the entire Gir Forest—and no time at all to see why these lions became symbols of royalty and greatness. A tiger will hide in the forest unseen, but a lion stands its ground, curious and unafraid lionhearted. Though they told me in subtle ways when I got too close, Gir"s lions allowed me unique glimpses into their lives during my three months in the forest. It"s odd to think that they are threatened by extinction; Gir has as many lions as it can hold—too many, in fact. With territory in short supply, lions move about near the boundary of the forest and even leave it altogether, often clashing with people. That"s one reason India is creating a second sanctuary. There are other pressing reasons, outbreaks of disease or natural disasters. In 1994 a serious disease killed more than a third of Africa"s Serengeti lions—a thousand animals—a fate that could easily happen to Gir"s cats. These lions are especially vulnerable to disease because they descend from as few as a dozen individuals. "If you do a DNA test, Asiatic lions actually look like identical twins," says Stephen O"Brien, a geneticist (基因学家) who has studied them. Yet the dangers are hidden, and you wouldn"t suspect them by watching these lords of the forest. The lions display vitality, and no small measure of charm.Though the gentle intimacy of play vanishes when it"s time to eat, meals in Gir are not necessarily frantic affairs. For a mother and her baby lion sharing a deer, or a young male eating an antelope (羚羊), there"s no need to fight for a cut of the kill. The animals they hunt for food are generally smaller in Gir than those in Africa, and hunting groups tend to be smaller as well. What impressed the author most when he went to watch the lions in the Gir Forest
竞争性抑制作用的特点是
能使胰蛋白酶原激活的物质是
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