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. 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