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  3. The railroad industry could not have grown as large as it did without steel. The first rails were made of iron. But iron rails w

The railroad industry could not have grown as large as it did without steel. The first rails were made of iron. But iron rails w

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问题     The railroad industry could not have grown as large as it did without steel. The first rails were made of iron. But iron rails were not strong enough to support heavy trains running at high speeds. Railroad executives wanted to replace them with steel rails because steel was ten or fifteen times stronger and lasted twenty times longer. Before the 1870’s, however, steel was too expensive to be widely used. It was made by a slow and expensive process of heating, stirring and reheating iron ore.
    Then the inventor Henry Bessemer discovered that directing a blast of air at melted iron in a furnace would burn out the impurities that made the iron brittle. As the air shot through the furnace, the bubbling metal would erupt in showers of sparks. When the fire cooled, the metal had been changed, or converted to steel. The Bessemer converter made possible the mass production of steel. Now three to five tons of iron could be changed into steel in a matter of minutes.
    Just when the demand for more and more steel developed, prospectors discovered huge new deposits of iron ore in the Mesabi Range, a 120 long region in Minnesota near Lake Superior. The Mesabi deposits were so near the surface that they could be mined with steam shovels.
    Barges and steamers carried the iron ore through Lake Superior to depots on the southern shores of Lake Michigan and Lake Erie. With dizzying speed Gary, Indiana, and Toledo, Youngstown, and Cleveland, Ohio, became major steel manufacturing centers. Pittsburgh was the greatest steel city of ail.
    Steel was the basic building material of the industrial age. Production skyrocketed from seventy seven thousand tons in 1870 to over eleven million tons in 1900.
According to the passage, the railroad industry preferred steel to iron because steel was ______.
选项 A、cheaper and more plentiful
B、lighter and easier to mold
C、cleaner and easier to mine
D、stronger and more durable
答案D
解析 第一段第四句“because steel was ten or fifteen times stronger and lasted twenty times longer”，表明人们以钢代铁的理由是钢的强度更大些，更耐用，所以D选项正确。
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