Exposure to the radio frequently fields（射频场）generated by mobile phones does not cause head pain or increase blood pressure, according to a Norwegian study. Instead, people

who experience such symptoms（征兆）do so because they expect that they will occur, the findings suggested.

Dr Gunnhild Oftedal and colleagues at the Norway University of Science and Technology in Trondheim experimented on 17 subjects who “ regularly experienced pain or discomfort in the head during or shortly after mobile phone calls lasting between 15 and 30 minutes.”

The participants were tested during mobile phone radiofrequency exposure and sham exposure（假性辐射）, without knowing which sessions was which. Each session lasted 30 minutes. and 65 pairs of trials were conducted.

As reported in the medical magazine Cephalalgia, the subjects said they felt an increase in pain or discomfort during 68 per cent of all trials. The degree of not associated with the order of trials.

The researchers observed no significant correlations between actual exposures and the subjects’ reports of symptoms, and no effects of exposure on changes in heart rate or blood pressure.

Oftedal’s team concludes that the most likely explanation for the headaches and discomfort reported by the subjects “is that the symptoms are due to negative expectations.”

67. According to the report, people using mobiles feel affected by exposure to the radiofrequency fields because __________

A. they have merely imaginary expectations

B. some symptoms just occur in their body

C. there are negative effects produced by mobiles

D. radiofrequency generated by mobiles is too high

68. Which word in the report refers to the same as the underlined word “subjects”?

A. researchers     B. symptoms     C. trials    D. participants

69. Dr Gunnhild Oftedal and his colleagues _______.

A. find effects of exposure on changes in heart rate or blood pressure

B. test the participants in two different situations

C. feel an increase in pain or discomfort during most trials

D. conclude that the symptoms do result from the radiofrequency fields

70. We can infer from the report that _____.

A. Dr Gunnhild Oftedal and his colleagues are strongly against the use of mobiles

B. the subjects share the same discomfort in both mobile radiofrequency exposure and sham exposure

C. the subjects are told in advance which section they will be in and which order they will follow

D. Dr Gunnhild Oftedal and his colleagues fail to find the side effects caused by exposure to the radiofrequency fields