C# .NET ต้องการอ่านค่า XML Response ที่ได้จาก Web Service ต้องทำยังไงคะ

            WebRequest wr = WebRequest.Create("url");
            wr.Method = "POST";
            wr.ContentType = "application/x-www-form-urlencoded";
            wr.ContentLength = data.Length;

            Stream st = wr.GetRequestStream();
            byte[] b = System.Text.Encoding.UTF8.GetBytes(data);
            st.Write(b, 0, b.Length);
            st.Close();

            WebResponse wp = wr.GetResponse();
            
            Stream s = wp.GetResponseStream();
            StreamReader sr = new StreamReader(s);
            string a = sr.ReadToEnd();
            XElement xml = XElement.Parse(a);
            txtStatus.Text = (from f in xml.Elements("STATUS") select f.Value).FirstOrDefault();
            txtCredit.Text =  (from f in xml.Elements("DETAIL") select f.Value).FirstOrDefault();
            txtExpire.Text =  (from f in xml.Elements("SMID") select f.Value).FirstOrDefault();
            s.Close();