c# xmlhttp POST提取远程webservice数据

 public string cancel(string StudentID,string Mobile)
    {
    string data=null;
   data= @"<?xml version='1.0' encoding='utf-8'?>
<soap:Envelope xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' xmlns:xsd='http://www.w3.org/2001/XMLSchema' xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/'>
  <soap:Header>
    <SPInfo xmlns='https://211.138.113.176/xxtsync/Sync'>
      <SPID>'01'</SPID>
      <SPPassword>'B571B533872371AB85B1D0D02B22FEAA'</SPPassword>
    </SPInfo>
  </soap:Header>
  <soap:Body>
    <BusQX xmlns='https://211.138.113.176/xxtsync/Sync'>
      <StudentID>'11111111111'</StudentID>
      <Mobile>'1111111111'</Mobile>
    </BusQX>
  </soap:Body>
</soap:Envelope>";
        data = data.Replace("'", "\"");
MSXML2.XMLHTTP xmlhttp = new MSXML2.XMLHTTP();       
xmlhttp.open("POST", "https://211.138.112.38/xxtsync/sync.asmx", false,null,null);
xmlhttp.setRequestHeader("Content-Type", "text/xml; charset=utf-8");
xmlhttp.setRequestHeader("Content-Length", data.Length.ToString());
xmlhttp.setRequestHeader("SOAPAction", "https://211.138.113.176/xxtsync/Sync/BusQX");
int n=xmlhttp.send(data);
Byte[] b = (Byte[])xmlhttp.responseBody;//返回数据
string s = System.Text.ASCIIEncoding.UTF8.GetString(b, 0, b.Length);
return n;
    }