How do dropdownlist which change from another dropdownlist asp.net C# xml

I have a Xml file:

  <Printer>
          <floor> 1 </floor>
          <room> 10</room>
          <name> Hp laserjet 1022 </name>
          <path> \\server\Hp </path>
     </Printer>
  <Printer>
          <floor> 2 </floor>
          <room> 22</room>
          <name> Canon Lbp 2030 </name>
          <path> \\server\Canon </path>
  </Printer>

I add three dropdownlist on webform. In the first dropdownlist be floor. For example: 1) When user select 1st floor in first ddl 2) In second dropdawnlist load all rooms on 1 floor, then user select room and in third dropdownlist load all printers on room.

Please! Give me a simple example how to do it. Sorry for my terrible English