He had already walked three or four miles ________ he saw a cart, half-full of hay, by the side of the road．

1. A.
   while
2. B.
   when
3. C.
   the moment
4. D.
   unless

He had already walked three or four miles ________ he saw a cart, half-full of hay, by the side of the road.

A. when             B. while                   C. until                    D. unless

He had already walked three or four miles        he saw a cart, half full of hay, by the side of the road.

A．while        B．when        C．the moment      D．unless