单选题 Earth Rocks on Most of the time, the ground feels solid beneath our feet. That's comforting. But it's also misleading because there's actually a lot going on underground. Masses of land (called plates) slip, slide, and bump against each other, slowly changing the shape of continents and oceans over millions and billions of years. Scientists know that Earth formed about 4.5 billion years ago. They also know that our planet was hot at first. As it cooled, its outermost layer, called the crust (地壳), eventually formed moving plates. Exactly when this shift happened, however, is an open question. Now, an international group of researchers has an answer. They've found new evidence suggesting that Earth's crust started shifting at least 3.8 billion years ago. The new estimate is 1.3 billion years earlier than previous ones. Not long before 3.8 billion years ago, lots of asteroids (小行星) were hitting Earth, keeping its crust in a hot, melted state. After the hard crust formed, much of it sank at various times into the planet's hot insides. There, it melted before returning to the surface. In some places, however, the crust never sank. One of the oldest such places is in Greenland, in an area called the lsua supracrustal (上地壳) belt. The rocky crust there is between 3.7 and 3.8 billion years old. The belt was once part of the seafloor, but now it is exposed to air. The researchers recently took a close look at the lsua supracrustal belt. They noticed long, parallel cracks in the rock that have been filled in with a type of volcanic rock. To explain this structure, the scientists propose that tension in the crust caused the seafloor to crack open long ago. Hot, liquid rock, called magma (岩浆), flowed up slowly from deep inside Earth to fill the cracks. Finally, the whole area cooled, forming what we see today. That explanation, plus chemical clues inside the rock, suggests that the lsua supracrustal belt was once part of a plate under the ocean, beginning around 3.8 billion years ago.

单选题A gift to the United States from France, the Statue of Liberty was assembled and dedicated in 1886.

单选题The carelessnesshas somebad effects on his showA. scoresB. resultsC. influencesD. luck

单选题The new credit card that is being introduced by Greenpeace is net made of plastic.

单选题It's no using trying to speak to her this morning: she's Uin a mood/U.

单选题Have you talked to her lately? A. lastly B. finally C. shortly D. recently

单选题The chemical is deadly to rats but safe to cattle.

单选题Cheating deserves our harshest condemnation .

单选题Mother Nature Shows Her Strength Tornadoes(龙卷风)and heavy thunderstorms moved across the Great Lakes and into Trumbull County on Saturday evening. The storms were dramatic and dangerous. George Snyder was driving the fire truck down Route 88 when he first noticed that a funnel (漏斗状的)cloud was behind him. "I stopped the truck and watched the funnel cloud. It was about 100 feet off the ground and I saw it go up and down for a while. It was moving toward Bradley Road and then suddenly it disappeared. "Snyder said. Snyder only saw one of the funnel clouds that passed through northeastern Ohio on Saturday. In Trumbull County, a tornado turned trees onto their sides. Some trees fell onto houses and cars. Other trees fell into telephone and electrical wires as they went down. Amanda Sym check was having a party when the storm began. "I knew something was wrong, "he said. "I saw the sky go green and pink(粉红色). Then it sounded like a train rushing toward the house. I started crying and told everyone to go to the basement for protection. " The tornado causeda 10 to damage to cars and houses in the area. It will take a longtime and much money to repair everything. There was also serious water damage from the thunderstorms. The heavy rains and high wind caused the power to go out in many homes. The storms caused serious flooding in areas near the river. More than four inches of rain fell in parts of Trumbull County. The river was so high that the water ran into streets and houses. Many streets had to be closed to Cars and trucks because of the high water. This made it difficult for fire trucks, police Cars, and other rescue vehicles to help people who were in trouble. Many people who live near the river had to leave their homes for their own safety. Some people reported five feet of water in their homes. Local and state officials opened emergency shelters for the people who were evacuated(撤走). The Red Cross served meals to them. "This was a really intense storm, "said Snyder. "People were afraid. Mother Nature can be fierce. We were lucky this time. No one was killed/

单选题 Genetically Modified Foods If you want to spark a heated debate at a dinner party, bring up the topic of genetically modified foods. For many people, the concept of genetically altered, high-tech crop production raises all kinds of environmental, healthy, safety and ethical questions. Particularly in countries with long a grain traditions-and vocal green lobbies—the idea seems against nature. In fact, genetically modified foods are already very much apart of out lives. A third of the cornand more than half the soybeans and cotton grown in the U.S. last year were the product of biotechnology, according to the Department of Agriculture. More than 65 million acres of genetically modified crops will be planted in the U.S. this year. The genetic genie (妖怪) is out of the bottle. Yet there are clearly some very real issues that need to be resolved. Like any new product entering the food chain, genetically modified foods must be subjected to rigorous testing. In wealthy countries, the debate about biotech is tempered by the fact that we have a rich array of foods to choose from. and a supply that far exceeds our needs. In developing countries desperate to feed fast-growing and underfed populations, the issue is simpler and much more urgent: Do the benefits of biotech outweigh the risks? The statistics on population growth and hunger are disturbing. Last year the world's population reached 6 billion. The U.N. estimates that nearly 800 million people around the world are undernourished. The effects are devastating. About 400 million women of child-bearing age are iron deficient, which means their babies are exposed to various birth defects. As many as 100 million children suffer from vitamin A deficiency, a leading cause of blindness. How can biotech help? Biotechnologists have developed genetically modified rice that is fortified with beta-carotene-which the body converts into vitamin A and additional iron, and they are working on other kinds of nutritionally improved crops. Biotech can also improve farming productivity in places where food shortages are caused by crop damage attributable to pests, drought, poor soil and crop viruses, bacteria or fungi.

单选题Mary is {{U}}looking for{{/U}} the book she lost yesterday A. trying to find B. trying to read C. trying to buy D. trying to borrow

单选题Many of Europe"s airports are heavily congested .

单选题The trumpet was part of the opera orchestra long before it became a standard orchestral instrument.

单选题Tonight"s live coverage of the hockey (曲棍球) game is very wonderful.

单选题How do you interpret the sentence 'her airplane Electra is not pressurized' (Para. 4)?

单选题They had a far better {{U}}yield{{/U}} than any other farm miles around this year.

单选题Our arrangements were thrown into complete turmoil .

单选题The leaves have been swept into huge {{U}}heaps {{/U}}.

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

单选题The phobia may have its root in a childhood trauma .