_______ the boy was walking down the street，the UFO landed．

A．Why          B．Where            C．While          D．Whenever

________ the boy was walking down the street，the UFO landed．

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________ the boy was walking down the street，the UFO landed．

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         tile boy was walking down tile street，the UFO landed．

A．While           B．After             C．Before             D．ago

 While the boy ______ down the street, the UFO _____ .

A．was walking; was landing

B．walked; landed

C．was walking; landed

D．walked; was landing