中文XML论坛--[求助]xpath exception

我的 xml文件aa.xml
<?xml version="1.0"?>
<?mso-application progid="Excel.Sheet"?>
<Workbook xmlns="urn:schemas-microsoft-com:office:spreadsheet"
xmlns:o="urn:schemas-microsoft-com:office:office"
xmlns:x="urn:schemas-microsoft-com:office:excel"
xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet"
xmlns:html="http://www.w3.org/TR/REC-html40">
<DocumentProperties xmlns="urn:schemas-microsoft-com:office:office">
  <Created>1996-12-17T01:32:42Z</Created>
  <LastSaved>2000-11-18T06:53:49Z</LastSaved>
  <Version>11.6568</Version>
</DocumentProperties>
<OfficeDocumentSettings xmlns="urn:schemas-microsoft-com:office:office">
  <RelyOnVML/>
  <RemovePersonalInformation/>
</OfficeDocumentSettings>
<ExcelWorkbook xmlns="urn:schemas-microsoft-com:office:excel">
  <WindowHeight>4530</WindowHeight>
  <WindowWidth>8505</WindowWidth>
  <WindowTopX>480</WindowTopX>
  <WindowTopY>120</WindowTopY>
  <AcceptLabelsInFormulas/>
  <ProtectStructure>False</ProtectStructure>
  <ProtectWindows>False</ProtectWindows>
</ExcelWorkbook>
<Styles>
  <Style ss:ID="Default" ss:Name="Normal">
   <Alignment ss:Vertical="Bottom"/>
   <Borders/>
   <Font ss:FontName="宋体" x:CharSet="134" ss:Size="12"/>
   <Interior/>
   <NumberFormat/>
   <Protection/>
  </Style>
</Styles>
<Worksheet ss:Name="Sheet1">
  <Table ss:ExpandedColumnCount="2" ss:ExpandedRowCount="1" x:FullColumns="1"
   x:FullRows="1" ss:DefaultColumnWidth="54" ss:DefaultRowHeight="14.25">
   <Row>
    <Cell><Data ss:Type="Number">11</Data></Cell>
    <Cell><Data ss:Type="Number">33</Data></Cell>
   </Row>
  </Table>
  <WorksheetOptions xmlns="urn:schemas-microsoft-com:office:excel">
   <Selected/>
   <Panes>
    <Pane>
     <Number>3</Number>
     <ActiveRow>3</ActiveRow>
     <ActiveCol>1</ActiveCol>
    </Pane>
   </Panes>
   <ProtectObjects>False</ProtectObjects>
   <ProtectScenarios>False</ProtectScenarios>
  </WorksheetOptions>
</Worksheet>
<Worksheet ss:Name="Sheet2">
  <Table ss:ExpandedColumnCount="0" ss:ExpandedRowCount="0" x:FullColumns="1"
   x:FullRows="1" ss:DefaultColumnWidth="54" ss:DefaultRowHeight="14.25"/>
  <WorksheetOptions xmlns="urn:schemas-microsoft-com:office:excel">
   <ProtectObjects>False</ProtectObjects>
   <ProtectScenarios>False</ProtectScenarios>
  </WorksheetOptions>
</Worksheet>
<Worksheet ss:Name="Sheet3">
  <Table ss:ExpandedColumnCount="0" ss:ExpandedRowCount="0" x:FullColumns="1"
   x:FullRows="1" ss:DefaultColumnWidth="54" ss:DefaultRowHeight="14.25"/>
  <WorksheetOptions xmlns="urn:schemas-microsoft-com:office:excel">
   <ProtectObjects>False</ProtectObjects>
   <ProtectScenarios>False</ProtectScenarios>
  </WorksheetOptions>
</Worksheet>
</Workbook>
我编写一段代码，从该xml文件中查找Data ss:Type="Number"节点：

        String filepath = Server.MapPath("aa.xml");
        XmlDocument xmldoc = new XmlDocument();
        xmldoc.Load(filepath);
        XmlNamespaceManager manager = new XmlNamespaceManager(xmldoc.NameTable);
        manager.AddNamespace(string.Empty, "urn:schemas-microsoft-com:office:spreadsheet");
       manager.AddNamespace("o", "urn:schemas-microsoft-com:office:office");
        manager.AddNamespace("x", "urn:schemas-microsoft-com:office:excel");
        manager.AddNamespace("html", "http://www.w3.org/TR/REC-html40");
        manager.AddNamespace("x2", "http://schemas.microsoft.com/office/excel/2003/xml");
        manager.AddNamespace("ss", "urn:schemas-microsoft-com:office:spreadsheet");
        XmlNode xmlnd = xmldoc.SelectSingleNode("Workbook//Data[@ss:Type='Number']");
        if (xmlnd != null) { Response.Write("found"); }
        else Response.Write("not found");

在调试的时候，老是出错，错误为：Namespace Manager or XsltContext needed.This query has a prefix,variable,or user-defined function.
请各位大侠帮帮我


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