2．Before 1840architects and builders made buildings from stone，brick，or wood．The outside walls held up the weight of the building．A building that had more than a few stories became very heavy．A tall building，such as a castle，had very thick walls．
As time went by，architecture began to change．By 1850builders had begun to use a metal called iron for the fronts of some large buildings．Iron is lighter than stone and stronger than wood．People dig raw iron from the ground，but raw iron is not enough to use in buildings．Builders must use another type of iron called an alloy．
An alloy is a metal that contains different metals and other compounds，or mixtures of chemicals．The metals and compounds are melted and mixed together to form the alloy．To make an iron alloy，workers use a blast furnace．This machine creates great heat．It has a tall chamber that is lined with brick and open at the top．Workers put limestone，raw iron，and coke （a substance made from coal） into the top of the furnace．They force hot air into the bottom of the furnace．The hot air and a chemical reaction turn the materials into a liquid mass．Workers pour off waste liquid，called slag，and save the iron．Then they form the hot iron into large blocks called pigs，which can be formed into objects．This formed iron is called cast iron．
Cast iron is strong，but it still contains unwanted materials．People wanted to remove these impurities．To do this，they needed to heat the iron to very high temperature．Blast furnaces could not make that much heat．
In England in 1855，Sir Henry Bessemer found the solution．He forced air through liquid pig iron．The oxygen in the air mixed with the impurities in the iron in a process called oxidation．Oxidation creates heat．The heat turns impurities into slag，which is poured off so that only pure steel remains．Pure steel is much stronger than iron．The Bessemer process took place in an egg-shaped machine called a converter．With this process，workers began to make large amounts of steel quickly and cheaply．
By the late 1800s，many people lived in big cities．The price of land increased，one way builders could lower costs was to build taller buildings，which used less land．In Chicago in 1889，George A．Fuller completed the Tacoma Buildings，which had steel beams to hold up its weight．Soon people built more tall buildings with 10to 20stories．These were the first skyscrapers．
50．Alloys are important because theyC．
A．have no impurities
B．are the raw materials that people use to make iron
C．may be more useful than the separate metals in the alloy
D．contain different metals
51．The Bessemer process was a success because it producedB．
A．iron from steel
B．steel quickly and cheaply
C．less slag than a blast furnace
D．impurities in the iron
52．Sir Henry Bessemer probablyD．
A．avoided difficult problems
B．created slag into pure steel
C．was not familiar with metals such as steel
D．knew how to make cast iron
53．From the article，you can conclude that architects throughout historyA．
A．thought carefully about materials when they designed buildings
B．could not design creative buildings until steel was invented
C．disliked stone，brick，and wood
D．liked to design tall buildings
54．Which of the statements below expresses the main idea of the article？A
A．Discoveries about iron and steel have allowed people to build taller buildings．
B．Sir Henry Bessemer developed a process to make steel．
C．Architects use many kinds of materials in buildings．
D．Changes about designing building．
分析 本文是科技类说明文；文中介绍了在历史上为了建筑高大的建筑，建筑墙所用的材料不断演变，由最初的木头、石头和砖块到后来的铁，使用的材料越来越结实．最终又通过不断地改进把铁炼制成钢铁；随着城市的扩展，城市的土地越来越紧缺，需要高楼，因此钢梁被用来建筑摩天大楼．
解答 50．C．推断题．第三段具体介绍合金的形成过程，推断人们通过复杂的过程把不同的金属和组成成分熔合成合金是因为合金比其中的金属有用，故答案选择C．
51．B．细节理解题．根据第5段的最后一句"With this process，workers began to make large amounts of steel quickly and cheaply．（用这个程序，工人们可以快速便宜地生产钢铁）"可知答案选择B．
52．D．推断题．由第4 段可知cast iron（生铁）里面还存在杂质，第5 段告诉我们这个问题由Sir Henry Bessemer 解决，由此推断他知道如何制作生铁，故答案选择D．
53．A．推断题．文章第一段告诉我们，为了支撑建筑的重量，建筑墙的材料在不断改变；由此推断在整个历史上，人们设计建筑时会仔细考虑材料．
54．A．主旨大意题．文章以建筑墙的材料的不断演变引出钢铁，并介绍如何炼制钢铁，在最后一段说明钢梁（steel beams）在高楼建筑中的运用；可见文章围绕钢铁的发现和在高楼建筑中的使用，故答案选择A．
点评 阅读要在整体理解文章的基础上，根据题干选项定位细节，然后对照选项做出合理的判断．如果是主旨大意的判断要把握文章整体，避免以偏概全．
