秦汉时期创造了辉煌灿烂的东方文明，科技发展在世界上遥遥领先。下列属于秦汉时期科技成就的有（  ）  

①《九章算术》②《齐民要术》③造纸术的发明和改进④全身麻醉手术法

    Everybody hates it, but everybody does it. A recent report said that 40%of Americans hate tipping. In America alone, tipping is a $ 16 billion-a-year industry. Consumers acting politely ought not to pay more than they have to for a given service. Tips should not exist. So why do they? The common opinion in the past was that tips both rewarded the efforts of good service and reduced uncomfortable feelings of inequality. And also, tipping makes for closer relations. It went without saying that the better the service, the bigger the tip.

    But according to a new research from Cornell University, tips no longer serve any useful function. The paper analyzes numbers they got from 2,547 groups dining at 20 different restaurants. The connection between larger tips and better service was very weak. Only a tiny part of the size of the tip had anything to do with the quality of service.

    Tipping is better explained, by culture than by the money people spend. In America, the custom came into being a long time ago. It is regarded as part of the accepted cost of a service. In New York restaurants, failing to tip at least 15% could well mean dissatisfaction from the customers. Hairdressers can expect to get l5%-20%, and the man who delivers your fast food $ 2. In Europe, tipping is less common. In many restaurants the amount of tip is decided by a standard service charge. In many Asian countries, tipping has never really caught on at all. Only a few have really taken to tipping. According to Michael Lynn, the Cornell papers' author, countries in which people are more social or outgoing tend to tip more. Tipping may reduce anxiety about being served by strangers. And Mr. Lynn says, “In America, where people are expressive and eager to mix up with others, tipping is about social approval. If you tip badly, people think less of you. Tipping well is a chance to show off.”

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    LONDON—Terror struck in the heart of London on Thursday as explosions ripped through three subway trains and blasted(炸开)the roof off a crowded red double-decker bus. At least 37 people were killed and more than 700 wounded in the deadliest attack on the city since the blitz(闪电战)in World War II.

    British Prime Minister Tony Blair blamed Islamic extremists and said the bombings were designed to coincide with the opening in Scotland of a G-8 summit of the world's most powerful leaders. Foreign Secretary Jack Straw said the bombings—which came the day after London won the bid to host the 2012 Olympics--- have the “hallmarks(特点)of an al-Qaida-related attack”.

    Trapped passengers in the Underground railway threw themselves on the floor, some sobbing. As subway cars quickly filled with smoke, people used their umbrellas try to break the windows so that they could get air. Passengers emerged from the Underground covered with blood and soot(黑烟). On the street, in a light rain, buses ferried the wounded, and doctors used a hotel as a hospital.

     “I didn't hear anything, just a flash of light, people screaming, no thoughts of what it was. I just had to get out of the train,” said subway passenger Chris Randall, 28, who was hospitalized with cuts and burns to the face, the legs and hands.

     “It was chaos(混乱),” said Gary Lewis, 32, evacuated(撤离)from a subway train at King's Cross station. “The one haunting(萦绕)image was someone whose face was totally black( with soot) and pouring with blood.”

    Police said there had been no warning and that the blasts at three subway stations went off within 26 minutes, starting at 8:51 a.m. in an Underground train just outside the financial district. Authorities initially blames a power failure but realized it was a terror attack after the bus bombing near the British Museum at 9:47 a.m.—less than an hour after the first explosion.