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

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

They were surprised that a child could work out the problem____they themselves couldn't.

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

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

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

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

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

They were surprised that a child could work out the problem____they themselves couldn't.

A.once 　　B.while 　　C.then 　　D.if

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

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

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

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

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

   A. should    B. would    C. could    D. might