The environment contains three major natural sources of radiation：radioactive elements in mineral deposits，uhraviolet light from the Sun，and cosmic rays．The carcinogenic potential of radioactive elements and ultraviolet light has been established，while that of highly energetic cosmic radiation remains to be documented．Clearly，chronic exposure to intense sunlight is fl major cause of skin cancer in humans；incidence is high in farmers，sailors，and habitual sunbathers．Since the most effective natural screen for uhraviolet light is the natural skin pigment，melanin，individuals with large amounts of melanin--blacks，for example--are resistant to the carcinogenic effects of uhraviolet light．Fair- complexioned people，on the other hand，are quite susceptible．It is important to point out that the term skin cancer，as it is generally used，includes not only malignant tumours of the nonpigmented cells of the skin but also tumours of the pigmented cells(melanoma)．Under the proper conditions ultraviolet light can cause cancer to develop in the very cells that produce the pigment that affords protection from solar radiation．①
The environment also contains dangerous ionizing radiation from artificial sources．These include X- rays used for medical diagnosis and therapy，radioactive chemicals，radioactive elements used in atomic reactors，and radioactive fallout arising from the testing of nuclear devices．Home appliances have been known to emit potentially harmful X-rays under certain circumstances，as was the case with soree colour television sets made during the l950s．。Radiation leaks from improperly constructed or operated microwave ovens can also occur，but the health hazards from such exposures have yet to be established． Certain medical applications of X-rays，such as those used to establish the size and position of a fetus before birth and to control acne，have been largely abandoned because of the increased risk of cancer．o Modern technology has greatly reduced the risk of diagnostic X-rays，however．During therapeutic x- irradiation for the treatment of cancer。great care is taken to focus radiation on the tumour and to shield the adjacent normal body tissues from undue exposure．Laboratory workers who use radioactive chemicals are careful to prevent undue exposure by contamination and to dispose of radioactive wastes properly．The cancer risk from radioactive fallout resulting from the testing of atomic devices is virtually impossible to establish with certainty because of the large geographic areas affected．In general，such exposure is not massive，and immediate effects are probably not of high significance．The absorption by growing crops and farm animals of long-lived radioactive isotopes，such as strontium，from fallout may pose a threat to humans，however，because the crops and animals are a source of food．[432 words]
1．The underlined word“that”in the first paragraph refers t0______．
A．ultraviolet light
B．radioactive element
C．carcinogenic potential
D．cosmic ray
2．The first paragraph is mainly about______．
A．three major natural sources of radiation
B．a(chǎn) major cause of skin cancer
C．the carcinogenic effects of cosmic radiation
D．the association of environmental radiation with skin cancer
3．The author believes that______．
A．white people are more vulnerable to radiation than blacks
B．chronic exposure to intense cosmic radiation rarely causes cancer
C．the radiation from natural sources is more harmful than that from artificial ones
D．the radiation from natural sources is less harmful than that from artificial ones
4．According to this passage，______．
A．radiation leaks from improperly operated microwave ovens have little immediate effect on human health
B．radiation leaks from improperly operated microwave ovens have fatal effect on human health
C．the radiation emitted by some home electronic appliances may be potentially harmful
D．the cancer risk from all the medical applications of X-rays has been considerably reduced
5．It can be inferred from the passage that______．
A．even brief exposure to radioactive fallout will cause cancer
B．radioactive fallout may harm humans in the long run
C．the cancer risk from radioactive fallout has been ruled out
D．long-lived radioactive isotopes rarely produce significant effects on human heal
The environment also contains dangerous ionizing radiation from artificial sources．These include X- rays used for medical diagnosis and therapy，radioactive chemicals，radioactive elements used in atomic reactors，and radioactive fallout arising from the testing of nuclear devices．Home appliances have been known to emit potentially harmful X-rays under certain circumstances，as was the case with soree colour television sets made during the l950s．。Radiation leaks from improperly constructed or operated microwave ovens can also occur，but the health hazards from such exposures have yet to be established． Certain medical applications of X-rays，such as those used to establish the size and position of a fetus before birth and to control acne，have been largely abandoned because of the increased risk of cancer．o Modern technology has greatly reduced the risk of diagnostic X-rays，however．During therapeutic x- irradiation for the treatment of cancer。great care is taken to focus radiation on the tumour and to shield the adjacent normal body tissues from undue exposure．Laboratory workers who use radioactive chemicals are careful to prevent undue exposure by contamination and to dispose of radioactive wastes properly．The cancer risk from radioactive fallout resulting from the testing of atomic devices is virtually impossible to establish with certainty because of the large geographic areas affected．In general，such exposure is not massive，and immediate effects are probably not of high significance．The absorption by growing crops and farm animals of long-lived radioactive isotopes，such as strontium，from fallout may pose a threat to humans，however，because the crops and animals are a source of food．[432 words]
1．The underlined word“that”in the first paragraph refers t0______．
A．ultraviolet light
B．radioactive element
C．carcinogenic potential
D．cosmic ray
2．The first paragraph is mainly about______．
A．three major natural sources of radiation
B．a(chǎn) major cause of skin cancer
C．the carcinogenic effects of cosmic radiation
D．the association of environmental radiation with skin cancer
3．The author believes that______．
A．white people are more vulnerable to radiation than blacks
B．chronic exposure to intense cosmic radiation rarely causes cancer
C．the radiation from natural sources is more harmful than that from artificial ones
D．the radiation from natural sources is less harmful than that from artificial ones
4．According to this passage，______．
A．radiation leaks from improperly operated microwave ovens have little immediate effect on human health
B．radiation leaks from improperly operated microwave ovens have fatal effect on human health
C．the radiation emitted by some home electronic appliances may be potentially harmful
D．the cancer risk from all the medical applications of X-rays has been considerably reduced
5．It can be inferred from the passage that______．
A．even brief exposure to radioactive fallout will cause cancer
B．radioactive fallout may harm humans in the long run
C．the cancer risk from radioactive fallout has been ruled out
D．long-lived radioactive isotopes rarely produce significant effects on human heal