Cigarette smoke has been shown to contain numerous #pounds that are known to cause cancer in experimental animals and that appear to be strongly linked to human cancer，especially cancer of the lun9.In addition，tobacco smoke has been implicated in the causation of cancer of the mouth，and to a Iesser extent the esophagus，pancreas，biliary system，and urinary bladder.Cigarette smoke also has been shown to contain a number of cocareinogens，substances that appear to enhance the effect of carcinogens when they are administered con#itantly.①
    The effect of environmental pollution can best be illustrated by describing the phenomenon of bioconcentration and its impact on the food chain.Widely used insecticides。which metabolically degrade very slowly and are highly soluble in fat，are a case in point.o After being applied on land for agricultural uses，such #pounds are washed by rains into streams，rivers，and lakes，where they are ingested by microscopic life forms，which serve as food for fish that are，in turn，the major food source for larger fish and aquatic birds.Since these #pounds are soluble in fat，after being ingested by an animal they are stored and concentrated in the animal's body fat.Repeated feeding eventually leads to high concentrations of the #pounds in the animal's body。so that its subsequent ingestion by a larger predator，perhaps by humans，presents the predator with a significant level of the #pound.④This can pose a health hazard so serious that，for example，fishermen on a contaminated lake are warned not to eat the fish they catch.The magnitude of the problem is illustrated by the insecticide DDT，high levels of which persist in the environment and in the bodies of humans，despite the fact that its use was restricted worldwide by the early l 970s.
    Food additives are another source of environmental chemicals that has caused concern.Although these have been the object of dispute and have given rise to the“natural food”fad。there is no evidence that food additives cause human cancer.In fact，some food additives，especially those that protect foods from be#ing rancid，have actually been shown to prevent chemically induced cancer in experimental animals. An additive that remains a matter of concern，however，is sodium nitrite，which is widely used to preserve processed meats. It has been shown that nitrite can react in the stomach with amines'which arise from the digestion of meat，to form nitrosamines，a group of #pounds that are potent carcinogens for certain laboratory animals.These #pounds are formed in such minute amounts in the stomach that some researchers doubt that they pose a significant carcinogenic hazard for humans. Similarly'the demonstration that certain #pounds formed by the burning of meat are carcinogenic for animals must be placed in proper perspective.Carcinogenesis experiments in animals usually require continuous exposure to high levels of chemicals to obtain statistically valid results in their relatively short life span of a year or tw0；extrap01ation of these results to the effect on human health should be approached most carefully. [506 words]
    1.It can be inferred from the text that______.
    A.cigarette smoke is the sole cause of lung cancer
    B.the cancer of the urinary bladder is mainly caused by cigarette smoke
    C.the cancer risk of smoking is primarily due to the carcinogens in cigarette smoke
    D.the cancer risk of smoking is primarily due to the cocarcinogens in cigarette smoke
    2.It is TRUE that______.
    A.cigarette smoke is #posed of both carcinogens and cocarcinogens
    B.cocarcinogens are more harmful than carcinogens
    C.cocarcinogens never exist in the absence of carcinogens
    D.carcinogens are nothing but substances that cause cancer
    3.It seems that insecticides______.
    A.will still be used for a foreseeable future
    B.will be restricted worldwide soon
    C.will soon be forbidden to use in agriculture
    D.should not be applied on land for agricultural uses
    4.The second paragraph is mainly about______
    A.the threats posed by environmental pollution to humans
    B.the health hazard posed by some widely used insecticides
    C.the food chain from microscopic lire forms to humans
    D.the harmful effects of DDT on human bodies
    5.According to this passage，______
    A.no food additives are carcinogens
    B.sodium nitrite is a potent carcinogen for both humans and animals
    C.most food additives can prevent chemically induced cancer in humans
    D.carcinogenic #pounds for animals may pose little carcinogenic hazard for humans [NextPage]     难句透析
    ①Cigarette smoke also has been shown tO contain a number of cocarcinogens，substances[that appear tO enhance the effect of carcinogens（when they are administered con#itantly）].
    【结构】在本句中，“substances”是“cocarcinogens”的同位语。方括号所标示的部分是“substances”的后置定语从句。圆括号所标示的部分是其中的状语从句。“they”指“coearcinogens”。
    【释义】 已经表明，香烟中还含有多种辅致癌物。当并存的辅致癌物同时起作用时，会增强致癌物的致癌效果。
    ②Widely used insecticides，[which metabolically degrade very slowly and are highly soluble in fat]，are a case in point.
    【结构】非限制性定语从句“which…in fat”修饰“insecticides”；过去分词“widely used”用做定语，修饰“insecti cides”；“a case in point”的意思是：“一个适例”。
    【释义】广泛应用的杀虫剂就是一个例证。这些杀虫剂新陈代谢退化极慢，极易融于脂肪。
    ③Repeated feeding eventually leads tO high concentrations of the #pounds in the animal'S body，[so that its sub—sequent ingestion by a larger predator，perhaps by humans，presents the predator with a significant level of the #— pound].
    【结构】“Repeated feedin9”是主句的主语；在“so that”引导的结果状语从句中，“its subsequent ingestion”是主语。“presents…with…”是谓语动词。
    【释义】反复摄入这些化合物最终导致其在动物体内高度积聚。更大的捕食者，或许是人类，后来捕食这些动物，造成这种化合物在捕食者体内的含量过高

    全文翻译
    实验显示，抽香烟所摄取的物质中有多种化合物能在实验所用的动物身上引发癌症，并且这些化合物也和人类的癌症有着密切的联系，特别是肺癌。除此之外，抽草烟则可能引发口腔癌，并在一定程度上会引发食道、胰腺、胆汁系统以及膀胱等部位的癌变。已经表明，香烟中还含有多种辅致癌物。当并存的辅致癌物同时起作用时，会增强致癌物的致癌效果。
    环境污染所带来的后果可以通过对物种密集以及其对食物链的影响的描述中得到最好的体现。广泛应用的杀虫剂就是一个例证。这些杀虫剂新陈代谢退化极慢，极易融于脂肪。在农业生产中施放了杀虫剂后，化合物就会被雨水冲刷进小溪、河流和湖泊里，在这里它们为微生物所吸收，之后又因为微生物成为鱼的食物而进入鱼的体内，再后来小鱼成为大鱼和水食性鸟类的食物。由于这些化合物是易融于脂肪的，在被动物吸收之后它们就在动物体内的脂肪当中储存堆积起来了。反复摄入这些化合物最终导致其在动物体内高度积聚。更大的捕食者，或许是人类，后来捕食这些动物，造成这种化合物在捕食者体内的含量过高。这会带来严重的健康灾难，比如说，渔民如果在受污染的湖中捕鱼那么他们都被警告不能食用这些鱼。问题的严重程度可由名为DDT的杀虫剂的例子来说明，全世界在20世纪70年代初就已经严格限制了这种杀虫剂的使用，但是现在人类的体内还是存在高浓度的该种杀虫剂。
    食物添加剂是另一类引起人们关注的环境化学物质。虽然这类物质一直都争议不断并且引发了“天然食品”的热潮，但是目前没有证据证明食品添加剂能引发人类的癌症。事实上，一些食物添加剂，特别是那些用于防治食品腐烂变酸的成份，在实验动物的身上都显示可以预防化学制剂引发的癌症。但是仍有一种添加剂值得注意，那就是钠化亚硝酸盐，它被广泛用于保存加工后的肉制品。实验显示亚硝酸盐和肉类被消化后产生的胺会发生反应，产生亚硝胺，而这种化合物对某些种类的实验用动物都具有潜在的致癌危险。这些化合物在胃中存在的量极其微少，以至于一些研究者都质疑其是否会对人类带来致癌危险。与此类似的，在证明肉类燃烧后产生的某些化合物对动物具有致癌作用的实验也应当被放到恰当的情景中来看待。以动物为目标的致癌实验通常需要将目标暴露在化学成分较高的环境中，以使得能在其相对较短的一至两年的短暂生命当中获取有效的统计数据；将这些结果外推至人类身上则需要慎之又慎。
    超纲词汇
    Additive adj.／n.附加的，添加剂
    amine n.胺
    Aquatic 水的，水生的，水栖的
    biliary adj.胆汁的，输送胆汁的
    Bioconcentration 生物浓度
    carcinogenesis 癌发生，致癌作用
    cocarcinogen n.助致癌原，辅致癌物
    con#itant n.伴随的，伴随物
    extrapolation n.外推法，推断
    esophagus n.食道
    implicate n.使牵连其中，含义，暗示，包含的东西
    ingest vt.摄取，咽下，吸收
    insecticide n.杀虫荆
    metabolic 代谢作用的，新陈代谢的
    nitrite n.亚硝酸盐
    nkrosamine n.亚硝胺
    pancreas 胰腺
    potent 有力的，有效的
    predator 掠夺者，食肉动物
    rancid 像油脂腐臭味的，腐臭的
    sodium n.钠
    参考答案：1.C 2.D 3.A 4.A 5.D