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已知矩阵A=(α1，α2，α3，β1)，B=(α3，α1，α2，β2)都是4阶矩阵，其中α1，α2，α3，β1，β2均是4维列向量．若|A|=1，|B|=2，则|A-2B|=______．
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Nearly all weather occurs in the lowest layer of the _____.
在结构化分析方法中用状态—迁移图表达系统或对象的行为。在状态—迁移图中，由一个状态和一个事件所决定的下一状态可能会有 (28) 个。
Is there enough oil beneath the Arctic National Wildlife Refuge (ANWR)to help secure America’s energy future President Bush certainly thinks so. He has argued that tapping ANWR’s oil would help ease California’s electricity crisis and provide a major boost to the country’s energy independence. But no one knows for sure how much crude oil lies buried beneath the frozen earth, with the last government survey, conducted in 1998, projecting output anywhere from 3 billion to 16 billion barrels. The oil industry goes with the high end of the range, which could equal as much as 10% of U.S. consumption for as long as six years. By pumping more than 1 million barrels a day from the reserve for the next two to three decades, lobbyists claim, the nation could cut back on imports equivalent to all shipments to the U.S. from Saudi Arabia. Sounds good. An oil boom would also mean a multibillion-dollar windfall(意外之才) in tax revenues, royalties (开采权使用费)and leasing fees for Alaska and the Federal Government. Best of all, advocates of drilling say, damage to the environment would be insignificant. "We’ve never had a documented case of an oil rig chasing deer out onto the pack ice," says Alaska State Representative Scott Ogan. Not so fast, say environmentalists. Sticking to the low end of government estimates, the National Resources Defense Council says there may be no more than 3.2 billion barrels of economically recoverable oil in the coastal plain of ANWR, a drop in the bucket that would do virtually nothing to ease America’s energy problems. And consumers would wait up to a decade to gain any benefits, because drilling could begin only after much bargaining over leases, environmental permits and regulatory review. As for ANWR’s impact on the California power crisis, environmentalists point out that oil is responsible for only 1% of the Golden State’s electricity output—and just 3% of the nation’s. We learn from the second paragraph that the American oil industry ______.
Our brains could be hard-wired to be male or female long before we begin to grow testes(丸)or ovaries(卵巢)in the womb. This discovery might explain why some people feel trapped in a body that’s the wrong sex, and could also lead to tests that reveal the true "brain sex" of babies born with ambiguous genitalia(生殖器). Till now, the orthodoxy among developmental biologists has been that embryos develop ovaries and become female unless a gene called SRY on the Y chromosome is switched on. If this gene is active, it makes testes develop instead. This switch is seen as the key event in determining whether a baby is a girl or a boy. Only after the gonads(性腺) form and flood the body with the appropriate hormones, the theory goes, is the sex of our minds and bodies determined. But in a study of mice, a team at the University of California, Los Angeles, has now found that males and females show differences in the expression of no fewer than 50 genes well before SRY switches on. "It’s the first discovery of genes differentially expressed in the brain, "says Eric Vilain, who led the UCLA team. "They may have an impact on the hard-wired development of the brain in terms of sexual differentiation independent of gonadal induction." Vilain is presenting details of seven of the 50 genes to the annual meeting of the American Society of Human Genetics in Baltimore this week. Three of these genes are dominant in females and four are dominant in males. The next step for Vilain and his team will be to show that the genes in question really do influence brain sexuality—and not just in mice. This is likely to be a much tougher proposition than merely showing there are differences in expression. But if the findings are confirmed, they could one day yield blood tests that allow doctors to establish the brain sex of babies born with genitalia that share features of both sexes. At present doctors and parents have to guess which gender to assign for surgical "correction". What has been found by the study at UCLA
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