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排泄性尿路造影绝对禁忌证为
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已知三元二次型xTAx的平方项系数均为0，并且α=(1，2，-1)T满足Aα=2α． (Ⅰ)求该二次型表达式； (Ⅱ)求正交变换x=Qy化二次型为标准形，并写出所用坐标变换； (Ⅲ)若A+kE正定，求k的值．
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. It can be learned from the passage that oil exploitation beneath ANWR’s frozen earth
I’d been living with my wife for eight years and one night. Morn says, "I guess you guys are never going to get married. I mean, you’ve been through jail together, you’re living together, but…, oh, forget it." "Oh, well," I said," put it like that and I’ll marry your daughter tomorrow." Actually, I don’t know what we were waiting for, except that for a guy it’s never the right time to get married. I’ m also suspicious of any two people who don’t struggle with that decision. Part of my problem was that I was still lusting in my heart after other ladies. But somehow I knew that I wasn’t going to find another woman remotely as great as my soon-to-be wife. It’s a good thing my mother-in-law finally spoke up. I finally gathered my courage one day when we were having a picnic, and popped the question. I also gave my wife a big tourist pamphlet about Switzerland. I wasn’t taking any chances. She said no. It killed me. I felt sick to my stomach. I lost my appetite. Our dog just stared at me, thinking, "If you’re not going to eat your lunch, I will." Finally, I said, "But the Switzerland trip is yours if you say yes." "Switzerland , "she said, "is filled with precise, humorless people." "Maybe I should have suggested Paris" For a minute it seemed as if my change in travel plans would rate a solid "maybe". But she said no again. When we woke up the next morning, she told me that she’d slept on my proposal. "I guess I was a little rude to you last night," she explained. Meanwhile, I’m figuring I’m off the hook for this marriage thing for at least another eight years. I could afford to be generous. "I asked, you said no. It’s okay," I said. I might have looked a little too relieved because later that day she gave me a little box. Inside was a gold watch. On the back was inscribed. "Yes. I’ve reconsidered." I liked the watch, so I did the right thing. The reason why the man had waited so long for his marriage was that he ______.
Scraps of food could soon be helping power your home, thanks to an ultra-cheap bacteria-driven battery. Its developers hope that instead of feeding the dog or making garden compost(混合肥料) ,organic household waste could top up your home’s electricity. Although such "microbial fuel cells" (MFCs)have been developed in the past, they have always proved extremely inefficient and expensive. Now Chris Melhuish and technologists at the University of the West of England(UWE)in Bristol have come up with a simplified MFC that costs as little as ￡10 to make. Right now, their fuel cell runs only on sugar cubes, since these produce almost no waste when broken down, but they aim to move on to carrot power. "It has to be able to use raw materials, rather than giving it a refined fuel," says Melhuish. Inside the Walkman-sized battery, a colony of E. coil bacteria produce enzymes that break down carbohydrates, releasing hydrogen atoms. The cell also contains chemicals that drive a series of redox, or reduction and oxidation reactions, stripping electrons from the hydrogen atoms and delivering them steadily to the fuel cell’s anode(正极). This creates a voltage that can be used to power a circuit. To prove the MFC works, the researchers are using it to power a small light-sensitive robot. And when a number of the cells are connected in series, they could power domestic appliances, running a 40-watt bulb for eight hours on about 50 grams of sugar. Earlier MFCs were inefficient because they relied on energy-hungry filters and pumps. By experimenting with different anode materials, the UWE team have figured out how to make their system work: they dump the bacteria and redox chemicals directly into the cell. In its current form, the UWE team says its organic battery can produce eight times as much power as any previous MFC. But Melhuish wants to improve this, both by scaling it up and finding a better mix of redox chemicals. Which of the following would be the most appropriate title for the passage
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