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  3. Asbestos (石棉)is tile generic name for a number of naturally occurring fibrous mineral silicates of which the most common types a

Asbestos (石棉)is tile generic name for a number of naturally occurring fibrous mineral silicates of which the most common types a

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问题     Asbestos (石棉)is tile generic name for a number of naturally occurring fibrous mineral silicates of which the most common types are crocidolite.also known as blue asbestos,and chrysotile (white asbestos) .
    Employed for at least 2,000 years，it is valued by industry because of its tensile strength and flexibility, and its resistance to acids,heat and friction. Asbestos has had a very wide variety of applications. It has been commonly employed in the building industry as a fireproofing agent and as a strengthener. The metal frames of buildings were sprayed with an asbestos solution to prevent the spread of fires,while asbestos was routinely mixed in with cement to provide greater resistance to weathering agents. It is also used in motor-vehicle brake linings,gas-mask filters,certain types of talcum power,fire-resistant clothing, corrugated-iron roofing and in water and air pipes. As a result of such wide employment，it is likely that most people have been exposed to at least a small quantity of asbestos fibers.
    During inhalation,all particles which enter the respiratory tract pass through a series of filter mechanisms. Particles are filtered out at different points of the respiratory tract depending on their size. The smaller the particle,the further into the respiratory it may penetrate before being arrested. The first set of filters consists of the hairs and mucons (粘液的)lining of the nose and mouth which prevent the entry of larger particles. When large particles are inhaled,they stimulate this mucous lining,which results in coughing and the consequent expulsion of the particles. The mucous lining extends downwards through hair-lined bronchial (支气管的)tubes of increasingly fine diameter which further filter the air before it reaches the respiratory bronchioles,a series of very fine tubes attached to air exchange chambers known as alveoli (肺泡). The alveoli are composed of a thin layer of ceils through which oxygen passes into blood vessels and is then distributed to the blood-stream.
    Particles smaller than eight micrometers may reach the alveoli. This is the case with blue asbestos particles,which are very straight and slender and tend not to be arrested by mucous and expelled. As a result,they may reach the alveoli and penetrate the delicate cells lining the wails.
Some asbestos fibrils (particles) which reach the alveoli may be surrounded by scavenger cells known as macrophages,which serve to remove the fibrils from the body by expectoration (吐痰)or excretion. Other particles may remain in the alveoli with no adverse effects in case where the asbestos particles have penetrated the alveolar wall,scar tissue develops. This reduces the effectiveness of the alveoli and so less oxygen passes into the blood and less carbon dioxide is removed from it. This condition is known as asbestosis(石棉沉滞症). The process of scarring may take place gradually and the disease may not be diagnosed until 20 or 30 year after the initial exposure.
    The passage below summarizes the main points of the passage. Read the summary and then select the best word or phrase from the box below, according to the passage. You should decide on the best choice and mark the corresponding letter on the Answer Sheet with a single line through the center.

    Due to its versatility,asbestos has been【B1】______since ancient times in a wide range of【B2】______When asbestos【B3】______are【B4】______they pass through a series of【B5】______which arrest their【B6】______The stimulation of【B7】______causes【B8】______which expels them from the respiratory tract, If particles are very【B9】______.which is the case with【B10】______asbestos,they may【B11】______as far as the alveoli. Some of these particles may【B12】______the alveolar wall and cause【B13】______to develop,This reduces the【B14】______of the lung and causes the condition known as 【B15】______.Symptoms of the disease may take up to【B16】______to appear.  
【B16】
选项
答案P
解析 通过“The process of scarring may take place gradually and the disease may not be diagnosed until 20 or 30 year after the initial exposure．”可知，空格处应填20 years。
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