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    20、Before the first non-stop flight made in 1949，it       necessary for all planes to land for refueling．

    A．had been    B．has been    C．would be  D．would have been

    20、A

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    Before the first non-stop flight made in 1949，it       necessary for all planes to land for refueling．

    A．had been    B．has been    C．would be  D．would have been

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