The boy was walking down the street         the UFO landed．

A．when    B．a(chǎn)fter             C．before             D．until

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

1. A.
   While
2. B.
   After
3. C.
   As
4. D.
   Before

_______ 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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