Cigarette smoke has been shown to contain numerous compounds 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 concomitantly．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 compounds 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 compounds 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 compounds 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 compound．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 becoming 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 compounds that are potent carcinogens for certain laboratory animals．These compounds 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 compounds 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 composed 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 compounds for animals may pose little carcinogenic hazard for humans
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 compounds 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 compounds 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 compounds 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 compound．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 becoming 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 compounds that are potent carcinogens for certain laboratory animals．These compounds 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 compounds 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 composed 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 compounds for animals may pose little carcinogenic hazard for humans