cascading dropdown list

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C#

XML

MVC

hi
I have 2 drop down lists, the first bound to a unit name and the second bound to a city name which are read from XML

public ActionResult Test()
        {

            string strXmlpath = Server.MapPath("~\\Models\\Location.xml");

            var unit=from u in XDocument .Load (strXmlpath ).Document .Descendants ("Unit") select u;
            List<SelectListItem> lst = new List<SelectListItem>();
            foreach (var u in unit)
            {
                if (u.Attribute("ID").Value == "2")
                {
                    var city = u.Elements("city");
                    foreach (var c in city)
                        lst.Add(new SelectListItem { Text = c.Attribute("Name").Value, Value = c.Attribute("ID").Value });
                }
               
            }

            ViewData["cites"] = lst;


            var model = new UnitViewModel
            {
                Units = from units in XDocument.Load(strXmlpath).Document.Descendants("Unit")


                        select new SelectListItem
                        {
                            Value = units.Attribute("ID").Value,
                            Text = units.Attribute("Name").Value
                        }
            };

            List<SelectListItem> items = new List<SelectListItem>();
            foreach (var v in model.Units)
            {
             

                    items.Add(new SelectListItem
                    {
                        Text = v.Text,
                        Value = v.Value
                    });
         
            }

            ViewData["xml"] = items;



            return View();
        }

but now when change the unit drop down list value,
the city dropdownlist should bind to the city Subset for the unit.

i know should use cascading drop down lists and json but i search in google and did not find reference or example which help me .
please help me?
EDIT: Fixed format, grammar.