Answersheet she = PaperAnalyseManager.GetAnswer(paperID, USERManager.GetUserById(userID).Name);
            string AnswerStr = she.Results;
            XmlDocument Xml = new XmlDocument();
            Xml.LoadXml(AnswerStr);            XmlElement eleProblem = (XmlElement)Xml.DocumentElement.SelectSingleNode("PR[@PID=" + problemID + "]");
            eleProblem.SetAttribute("SCORE", value);
            eleProblem.SetAttribute("SCORED", "1");
            
            Xml.Save(AnswerStr);AnswerStr="<PRESULT><PR PID="1" TYPE="3"><ANSWER MARKED="0"><MAIN /><SUBS><SUB><![CDATA[fdsfds]]></SUB></SUBS></ANSWER><RESULT RIGHT="0" SCORED="0" SCORE="0" /><TIME SPEND="0" /></PR><PR PID="28" TYPE="3"><ANSWER MARKED="0"><MAIN /><SUBS><SUB><![CDATA[fds]]></SUB></SUBS></ANSWER><RESULT RIGHT="0" SCORED="0" SCORE="0" /><TIME SPEND="0" /></PR><PR PID="47" TYPE="3">......";保存的时候失败  如何解决这个问题