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在某市为评价一种新疫苗接种的预防效果的研究中，接种疫苗组的发病率为4/10万，接受安慰剂组的发病率为8/10万，该疫苗的保护率为
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______ one has a child will one realize how great one's parents are.
某乡5000户约2万人口，欲抽其1/5人口进行结核感染状况调查，从该地区户籍管理部门得到人口名单进行编号，逢尾数为5的列为调查对象。这种抽样方法为
Text 2As the merchant class expanded in the eighteenth - century North American colonies, the silversmith and the coppersmith businesses rose to serve it. Only a few silversmiths were available in New York or Boston in the late seventeenth century, but in the eighteenth - century they could be found in all major colonial cities. No other colonial artisans rivaled the silversmiths’ prestige. They handled the most ex pensive material and possed direct connections to prosperous colonial merchants. Their products, primarily silver plates and bowls, reflected their exalted status and testified to their customers’ prominence.Silver stood as one of the surest ways to store wealth at a time before neighborhood banks existed. Unlike the silver coins from which they were made, silver articles were readily identifiable. Often formed to individual specifications, they always carried the silversmith’ s distinctive markings and consequently could be traced and retrieved.Customers generally secured the silver for the silver objects they ordered. They saved coins, took them to smiths, and discussed the type of pieces they desired. Silversmiths complied with these requests by melting the money in a small furnace, adding a bit of copper to form a stronger alloy, and casting the alloy in rectangular blocks. They hammered these ingots to the appropriate thickness by hand, shaped them, and pressed designs into them for adornment. Engraving was also done by hand. In addition to plates and bowls, some customers sought more intricate products, such as silver teapots. These were made by shaping or casting pans separately and then soldering them together.Colonial coppersmithing also came of age in the early eighteenth century and prospered in northern cities. 12opper’ s ability to conduct heat efficiently and to resist corrosion’ contributed to its attractiveness. But because it was expensive in colonial America, coppersmiths were never very numerous. Virtually all copper worked by smiths was imported as sheets or obtained by recycling old copper goods. Copper was used for practical items, but it was not admired for its beauty. Coppersmiths employed it to fashion pots and kettles for the home. They shaped it in much the same manner as silver or melted it in a foundry with lead or tin. They also mixed it with zinc to make brass for maritime and scientific instruments. According to the passage, which of the following eighteenth -century developments had a strong impact on silversmiths（）
The professor talked to American and Brazilian students about lateness in both an informal and a formal situation: lunch with a friend and in a university class, respectively. He gave them an example and asked them how they would (1) if they had a lunch appointment with a friend, the average American student (2) lateness as 19 minutes after the (3) time. On the other hand, the average Brazilian student felt the friend was late after 33 minutes.In an American university., students are expected to arrive at the appointed (4) Classes not only begin, but also end at the (5) time in the United States. In the Brazilian class, only a few students left the class at noon; many (6) past 12:30 to discuss questions. (7) arriving late may not be very important in Brazil, (8) is staying late. The (9) for these differences is complicated. People from Brazilian and North American (10) have different feeling about lateness. In Brazil, the students believe that a person who usually (11) than a person who is always (12) . In fact, ..Brazilians expect a person with (13) or prestige to arrive late, while in the United States lateness is usually (14) disrespectful and unacceptable. (15) , if a Brazilian is late for an appointment with a North American , the American may misinterpret the (16) and become angry.As a result of his study, the professor learned that the Brazilian students were not being (17) to him. Instead, they were simply be having in the (18) way for a Brazilian student in Brazil. Eventually, the professor was able to (19) his own behavior so that he could feel (20) in the new culture. 14（）
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