They were surprised that a four-year-old boy should work out such a difficult problem ____ they themselves couldn’t.

      A. once           B. then        C. while         D. if

They were surprised that a four-year-old boy should work out such a difficult problem ____ they themselves couldn’t.

      A. once          B. then               C. while           D. if

They were surprised that a four-year-old boy should work out such a difficult problem ____ they themselves couldn’t.

A. once　　　　 B. then　　　　　 C. while　　　　　 　　 D. if

They were surprised that a four-year-old boy should work out such a difficult problem ____they themselves couldn’t.

1. A.
   once
2. B.
   then
3. C.
   while
4. D.
   if

They were surprised that a four-year-old boy should work out such a difficult problem

____they themselves couldn’t.

  A.once        B.then        C.while      D.if