xml datasource and CascadingDropDown

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C#

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hey !
i have a project that has 4 dropdownlist !
first dropdownlist defines mainland !
well i want when user choose his/her mainland, the countries of the choosen mainland show on second dropdown list and then show states of choosen country in third dropdownlist also show cities in last dropdownlist !
i know must use ajax CascadingDropDown !
i saved the mainlands and countries ... in a xmlfile like below:

<?xml version="1.0" encoding="utf-8" ?>
<nations>
  <mainland Name="Asia">
    <country Namec="Iran">
      <states Names="Tehran">
        <city Nameci="Tehran"/>
      </states>
    </country>
  </mainland>
  <mainland Name="Europe">
    <country Namec="Armenia">
      <states Names="Yerevan">
        <city Nameci="Yerevan"/>
      </states>
    </country>
  </mainland>
  <mainland Name="North America">
    <country Namec="USA">
      <states Names="California">
        <city Nameci="Los Angeles"/>
      </states>
    </country>
  </mainland>
  <mainland Name="South America">
    <country Namec="Brazil">
      <states Names="Brazilia">
        <city Nameci="Brazilia"/>
      </states>
    </country>
  </mainland>
</nations>

my problem is what should enter on .asmx.cs file (web service file) to achieve my wish output ?

please help ! tnx all