Html.DropDownList selected value change

i have 2 html.dropdown list where one bind to xml file and show part and other dropdown show cites of this part .

now how when change part (change value of first dropdownlist) in other dropdownlist show cites of this part?开发者_高级运维

Populate dropdowns using this tutorial. Make User Controls (partial pages) for dropdowns. Call second partial using ajax on change event of the first dropdown