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  3. Radiation occurs from three natural sources, radioactive material in the environment, such as in soil, rock, or building materia

Radiation occurs from three natural sources, radioactive material in the environment, such as in soil, rock, or building materia

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问题     Radiation occurs from three natural sources, radioactive material in the environment, such as in soil, rock, or building materials; cosmic rays; and substances in the human body, such as radioactive potassium in bone and radioactive carbon in tissues. These natural sources account for an exposure of about 100 million a year for the average American.
    The largest single source of man-made radiation in medical X-rays, yet most scientists agree that hazards from this source are not as great as those from weapons-test fallout, since strontium-90 and carbon-14 become incorporated into the body, hence delivering radiation for an entire lifetime. (46)The issue is, however, by no means uncontroversial; indeed, the last two decades have witnessed intensified examination and dispute about the effects of low-level radiation.
    A survey conducted in Britain confirmed that an abnormally high percentage of patients suffering from arthritis of the spine who had been treated with X-rays contracted cancer. Another study revealed a high incidence of childhood cancer in cases where the mother had been given X-rays. (47)These studies have pointed to the need to reexamine the assumption that exposure to low linear energy transfer presented only a minor risk.
    Recently, examination of the death certificates of former employees of a West Coast plant which produces plutonium for nuclear weapons revealed markedly higher rates for cancers of the pancreas, lung, bone marrow and lymph systems than would have been expected in a normal population.
    (48)While the National Academy of Sciences committee attributes these differences to chemical or other environmental causes, rather than radiation, other scientists maintain that any radiation exposure, no matter how small, leads to an increase in cancer risk. (49)It is believed by some that a dose of one rem, if sustained over many generations, would lead to an increase of one percent in the number of 1,000 disorders per million births.
    In the meantime, regulatory efforts have been disorganized, fragmented, and inconsistent, characterized by internecine strife and bureaucratic delays. A Senate report concluded that coordination of regulation among involved departments and agencies was not possible because of jurisdictional disputes and confusion. (50)One Federal agency has been unsuccessful in its efforts to obtain sufficient funding and manpower for the enforcement of existing radiation laws, and the chairperson of a panel especially created to develop a coordinated Federal program has resigned.

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答案这些研究说明有必要重新审视一种假设，即暴露在低幅度辐射的危险性极小。
解析 注意本句的难点为宾语的修饰语太长，其中包含一个"that"引导的同位语。
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