XML 增、删、改、求平均数

string file = "c:\\work.xml";
        private void btnCearte_Click(object sender, EventArgs e)
        {
            XmlDocument doc = new XmlDocument();
            XmlDeclaration dl = doc.CreateXmlDeclaration("1.0", "Unicode", null);
            doc.AppendChild(dl);
            XmlElement @class = doc.CreateElement("class");
            doc.AppendChild(@class);
            doc.Save(file);
        }
        private void btnInsert_Click(object sender, EventArgs e)
        {
            XmlDocument doc = new XmlDocument();
            doc.Load(file);
            XmlElement student = doc.CreateElement("student");
            student.SetAttribute("age", "20");
            XmlElement name = doc.CreateElement("name");
            name.InnerText = "童小霞";
            XmlElement sex = doc.CreateElement("sex");
            sex.InnerText = "famale";
            student.AppendChild(name);
            student.AppendChil