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某地为了了解新研制的流感气雾免疫制剂的效果，在本地随机确定5000人接种疫苗，另外5000人不接种该疫苗作为对照，经过一个流行期后，前者50人发病，后者500人发病，则 该疫苗的保护率为
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用于确证人类致癌物的研究方法是
An airplane flies because its wings push down on the air flowing past them, and in reaction, the air pushes up on the wings. When an airplane is level or rising, the front edges of its wings ride higher than the rear edges. The angle the wings make with the horizontal is called the angle of attack. As the wings move through the air, this angle causes them to push air flowing under them downward. A sleeper angle of attack will cause the wings to push more air downward. The third law of motion voiced by English physicist Isaac Newton states that every action produces an equal and opposite reaction. In this case, the wings pushing air downward is the action, and the air pushing the wings upward is the reaction. This causes lift, the upward force on the plane, Lift is often explained using Bernoulli’s principle, which states that, in some circumstances, a faster moving fluid(流体) (such as air)will have a lower pressure than a slower moving fluid. The air on the top of an airplane wing is at a lower pressure than the air underneath the wing, but this is an effect of lift, not its cause.Lift is one of the four primary forces acting upon an airplane. The others are weight, thrust, and drag. Weight is the force that offsets lilt. because it acts in the opposite direction. The weight of the airplane must be overcome by the lift produced by the wings, if an airplane weighs 4.5 metric tons, then the lift produced by its wings must be greater than 4.5 metric tons in order for the airplane to leave the ground. Designing a wing that is powerful enough to lift an airplane off the ground, and yet efficient enough to fly at high speeds over extremely long distances, is one of the marvels of aircraft technology. What does the article tell us aboutWhy does an airplane fly
男，31岁，1年来疑被人迫害，数次出走后被家人找回。近半年来常行为诡秘，将家中闹钟、收音机等在半夜深埋在公园树林地下，对亲友冷淡，生活懒散，房间内藏有刀、棍等，体格检查无阳性发现。精神状况检查：接触被动，有自语自笑，衣着尚得体。其称“有人要谋害我”，“有人命令我要么自杀、要么逃出去”否认有病，无自知力 症状学诊断为
Directions:Read the following text. Choose the best word(s) for each numbered blank and mark A, B, C or D on ANSWER SHEET 1. Advances in computers and data networks inspire visions of a future "information economy" in which everyone will have (1) to gigabytes of all kinds of information anywhere and anytime, (2) information has always been a (3) difficult commodity to deal with, and, in some ways, computers and high-speed networks make the problems of buying, (4) , and distributing information goods worse (5) better. The evolution of the Internet itself (6) serious problems. (7) the Intemet has been privatized, several companies are (8) to provide the backbones that will carry traffic (9) local networks, but (10) business models for intereonnectinn—who pays how much for each packet (11) , for example—have (12) to be developed. (13) intereonnection standards are developed that make (14) cheap and easy to transmit information across independent networks, competition will (15) . If technical or economic (16) make interconnection difficult, (17) transmitting data across multiple networks is expensive or too slow, the (18) suppliers can offer a signfficant performance (19) ; they may be able to use this edge to drive out competitors and (20) the market.
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