occur 发生 Don't let this occur again. The Olymipcs occur every four years. It occurred to sb th——青夏教育精英家教网—

摘要： occur 发生 Don't let this occur again. The Olymipcs occur every four years. It occurred to sb that ... 人突然想到- It occurred to me that I forgot my pen in the classroom.

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　　Metals usually occur (发生,出现) in nature as minerals(矿石). At present, about 80 different metals are known to man. But with so many metals to choose from the question arises: which metal shall we use? Is there any metal that is “best for everything”? Of course not. For example, iron is strong but it rusts easily, while gold, one of the first metals found by man, does not rust at all. Should we use gold, therefore, to build fences? Imagine the cost of a fence made of gold! It’s important to consider the cost and the supply (提供)of a metal. Also, when we choose a metal for a certain job, we examine its physical properties. Is it strong enough to be used as a support for a bridge? Can it he used in a telephone to conduct (传导) electncity? Is it light enough to be used in an air plane wing? Can it he drawn out to make a fine wire? We also examine its chemical properties. Will it resist acid (酸) ? Is it too active to use? Is it easy to combine [ (使)结合] with other elements? Each use must be studied carefully. Then we can choose the right kind of metal, or combinations(结合)of metals.

　　The metals used for constructing buildings, cars, planes and ships must he strong and easy to form by casing, machining, and cutting. These metals are called structural(生锈)metals. The two most important structural (结构的) metals are iron (steel) and aluminum. Aluminum, which is much lighter than iron, has made possible the modem aviation (航空) industry.

　　Very few of the metal things we use in industry and every day life are made of a single pure metal. Most of them are combinations of metals called alloys (合金). Alloys are formed when two or more metals are melted together and allowed to cool and harden. It has been known for a long time that when some metals are combined in this way, the alloy takes on new physical properties (性质). The use of alloy makes it possible to manufacture (制造) many articles which must have special properties. Nowadays, if an inventor cannot find a metal that suits his purpose, he tries to have a new alloy made that will. The search for new alloys is going on all the time.

1．Metals are usually ________.

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A．best for everything

B．made use of in daily life in pure forms

C．found in nature as minerals

D．in the same quality

2．________ According to the text, uses of metals depend on their ________.

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