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    Only in the past 50 years, with the revolution in diving equipment, study deep seas closely. A. were scientists able to B. scientists were able to C. have scientists been able to D. scientists have been able to 【查看更多】

     

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    Only in the past 50 years, with the revolution in diving equipment, ________ study deep seas closely.


    1. A.
      were scientists able to
    2. B.
      have scientists been able to
    3. C.
      would scientists be able to
    4. D.
      will scientists be able to

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