Scientists are closely concerned with the structure of buildings and with the quality of building materials. The World health Organization (WHO) observes that the introduction of air conditioning and energy conservation (保护) measures have been accompanied by growing problems of indoor air quality. Some pollutants arise from insulation (隔音) products, some from moving cars, and others from modern housing materials. As many Europeans spend up to 90% of their lives in buildings, the health effects of the indoor climate are very important. Some construction materials, including fiberboard, insulation foams and certain glues for man-made wood floor boards, for example, give out organic products such as formaldehyde (甲醛). Heat and humidity increase the release of formaldehyde and the gas seriously harms the eyes. Paint, lacquer, etc. can also release dangerous gas into indoor air. Construction materials can cause serious damage, especially when they contain asbestos（石棉）. Asbestos is naturally present in rock formations worldwide. It belongs to a family of mineral substances made up of solid, non-combustible（非易燃品） fiber. These properties make asbestos a highly sought-after construction material. As early as 1931, however, public health officers in the United Kingdom revealed the connection between breathing in asbestos dust and such diseases as lung cancer. The land on which a building is sited may also contribute to pollution. Some kinds of granite or similar rocks contain traces of radium. As it breaks down, this naturally radioactive element produces some kind of radioactive gas that goes through tiny cracks in walls, floors and building materials, and makes its way into the building and the rooms. The better the homes are insulated, the more is the dangerous gas in indoor air. The main effect of this dangerous gas on health is to increase the risk of lung cancer. 53．What is the main idea of the passage? A．Some building materials pollute indoor air. B．Some factors cause indoor air pollution. C．Asbestos can cause lung cancer. D．The land on which houses are built contributes to indoor pollution. 54．Why are Europeans particularly concerned with building materials? A．Lots of building materials there are radioactive. B．They stay home up to 90% of their lives. C．They have a high rate of lung cancer. D．They spend most of their time indoors. 55．Why is asbestos a sought-after building material? A．It is a kind of insulation foam. B．Asbestos will not give out dangerous gas. C．This material is not easy to catch fire. D．It is rarely present in rock formations. 56．Why should we be careful about the land on which a building is to be sited? A．To determine whether the land is firm enough for a building to be sited on. B．To make sure that the land contains no radioactive material. C．To make certain that the land releases no formaldehyde or other gases. D．To check whether it contains any combustible materials.