B超机是利用超声波来诊断病情的仪器,但是人们却听不到它发出的

B超机是利用超声波来诊断病情的仪器,但是人们却听不到它发出的声音,这是因为   

    A.声音太大    B.声音的频率小于人耳能听到的最小频率

    C.声音太小    D.声音的频率大于人耳能听到的最大频率

答案

D

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