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;
}