They were surprised that a child should work out the problem________they themselves couldn't．

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

They were surprised that a child should work out the problem ________ they themselves couldn't.

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

They were surprised that a child should work out the problem _____ they themselves couldn’t.

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

They were surprised that a child should work out the problem _____ they themselves couldn’t.

1. A.
   when
2. B.
   where
3. C.
   until
4. D.
   while

They were surprised that a child should work out the problem they themselves couldn’t．

A．once B．then C．while D．if