Cellular phones work by transmitting radio waves to base stations that connect calls into a network. The waves are a form of non-ionizing(非离子的) radiation-unlike, say, X rays, which have the power to change the atoms in human cells to potentially hazardous ions. Non—ionizing radiation can also be dangerous. At the high levels found in radar or inside microwave ovens(微波炉), it can heat and severely damage human beings. The question for scientists is whether the low—energy (and low-heat) signals from cellular phones can do harm.

     Cancer studies have been inconclusive since 1993,when a Florida man brought an unsuccessful charge that blamed his wifes fatal brain tumor(肿瘤) on her use of a cellular phone. In a frequently quoted 1997 report, Australian researchers exposed mice to two daily 30-min. amount of cellular-phone radiation for up to 18 months. The mice developed tumors at twice the rate of animals that were radiation-free. But the results havent been widely quoted, and some scientists question their relevance. For now m the best advice science can offer about cellular phones is handle with care.

    21. According to the passage, science cannot offer us a definite connection between ________.

    [A] cellular-phone radiation and changes in brainwave patterns

    [B] X rays and the atoms in human cells

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