单选题 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 millirems a year for the average American.
The largest single source of man-made radiation is 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. The issue is, however, by no means uncontroversial. The last two decades have witnessed intensified examination and dispute about the effects of low-level radiation, beginning with the United Nations Scientific Committee on the Effects of Atomic Radiation, which reported in 1958 that "even the smallest amounts of radiation are likely to cause deleterious genetic and perhaps also somatic effects."
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 prenatal pelvic X rays. These studies have pointed to the need to reexamine the assumption that exposure to low-linear energy transfer presents only a minor risk.
Recently, examination of the death certificates of former employees of a West Coast plant that produces plutonium for nuclear weapons revealed markedly higher rates for cancers of the pancreas, lung, bone marrow, and lymphatic system than would have been expected in a normal population.
While the National Academy of Sciences committee attributes this difference 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. It is believed by some that a dose of one rem, if sustained over many generations, would lead to an increase of 1 percent in the number of serious genetic defects at birth, a possible increase of 1,000 disorders per million births.
In the meantime, regulatory efforts have been disorganized, fragmented, inconsistent, and 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. 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.

单选题 The primary purpose of the passage is to ______.

单选题 Which of the following, according to the passage, is a list of three natural sources of radiation?

单选题 Which of the following does the author cite in support of the quotation from the United Nations Scientific Committee on the Effects of Atomic Radiation?
I. Strontium 90 and carbon 14 become incorporated into the body and deliver radiation for an entire lifetime.
II. An abnormally high percentage of patients with arthritis of the spine who were treated with X rays subsequently contracted cancer.
III. A high incidence of cancer appeared among children of mothers who had been given prenatal pelvic X rays.

单选题 According to the passage, some scientists believe that a dose of one rem of radiation continued over a period of generations would ______.

单选题 It can be inferred from the last paragraph of the passage that the chairperson who resigned from the panel to develop a coordinated federal program for radiation regulation most likely did so because