题目

下列关于合金的说法正确的是 (    ) ①合金只能由两种或两种以上的金属组成 ②合金具有金属特性 ③碳钢和生铁的主要成分相同，故性能相同 ④改变原料的配比及生成合金的条件，可得到具有不同性能的合金 A． ②③ B． ②④ C． ①② D． ①④ 答案：B【详解】 ①合金可由两种或两种以上的金属或者金属与非金属熔合而成，故错误； ②合金是指在一种金属中加热熔合其他金属或非金属而形成的具有金属特性的物质，故正确； ③碳钢和生铁含碳量不同，含碳量越高，硬度越大，因此两者的性能不同，故错误； ④合金的性质与合金的原料配比以及　　The city of Venice, in Italy, is one of a kind. It is built on more than 120 islands, just off the coast. The islands are small and close together. The waterways between them are used as streets. They are called canals. 　　In this watery world, people use boats to get from place to place. Gondolas to place, small boats that are moved with one oar（桨）, were once the only boats used in the canals. But today motorboats make getting around faster; there are still lots of gondolas, though. Many visitors go to Venice all the year round. And they like to ride in the old-style（老式）boats. 　　If you aren't going far in this city, it's easier to go on foot. There are hundreds of narrow footpaths. About 100 low bridges connect the footpaths and islands. But these bridges cause problems for the boaters. When the sea tide（潮）is high, the canal water rises. People in the boats must duck to fit under bridges. (1)After reading the first paragraph, we can come to the conclusion that ________. [　　] A．all the buildings in Venice are help up（抬高）on seawater B．there's no land in the city of Venice C．the streets in Venice are all lined with waterways D．nobody can be seen walking in the streets of Venice (2)It is clear that ________. [　　] A．all the Venetian people ride in boats with one oar to get from place to place B．gondolas are the only boats used in the canals C．the ways of getting round in Venice are different from those in any other city D．motorboats have taken the place of gondolas in Venice now (3)Which of the following statements shows no difference between itself and the passage? ________ [　　] A．All the Venetian people like to ride in the old-style boats. B．Neither cars nor buses are used in Venice. C．Citizens in Venice don't walk much. D．Gondolas rides are much more expensive. (4)Which of the following statements leads us to the conclusion that traveling around Venice by gondolas can be slow? ________ [　　] A．Gondolas, small boats that are moved with one oar, were once the only boats used in the canals. B．But today, motorboats make getting around faster. C．But these bridges cause problems for the boaters. D．People in the boats must duck to fit under bridges. (5)The word“duck”in the last sentence means ________. [　　] A．bend one's head in order not to be hit B．go quickly under water for a short time C．swim as a duck does D．dive suddenly like a duck