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 My brother left home five years ago. I _______ nothing from him up to now.

  A. hear                                   B. heard                       C. have heard                D. had heard

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Nuclear power's danger to health, safety, and even life itself can be described in one word; radiation(辐射).

    Nuclear radiation has a certain mystery about it, partly because it cannot be detected (探测) by human senses. It can't be seen or heard, or touched or tasted, even though it may be all around us. There are other things like that. For example, radio waves are all around us but we can't detect them, sense them, without a radio receiver. Similarly, we can't sense radioactivity without a radiation detector. But unlike common radio waves, nuclear radiation is not harmless to human beings and other living things.

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