Multiple Dropdown <Select> & XML instance

Multiple Dropdown <Select> & XML instance

Hi All,

I'm currently trying to Pull data over from an XML feed, and display it Via ajax, Which is working and is fine, 

Apart from now, I'm trying to display the pulled data into dropdown boxes, which is fine, But I'm trying to make the second dropdown box only activate and display the relevant information when the top level Dropdown box has been selected, 

For example The XML Feed is as followed,

  <Course id="123">
   <Name>Course1</Name>
<instance>
<location>London</location>
<date>11/11/2011</date>
</instance>
<instance>
<location>London</location>
<date>11/11/2011</date>
</instance>
  </Course>
  
  <Course id="1234">
   <Name>Course2</Name>
<instance>
<location>London</location>
<date>11/11/2011</date>
</instance>
<instance>
<location>London</location>
<date>11/11/2011</date>
</instance>
  </Course>


When Course1 Is selected, In the second dropdown box I only want to display the 2 different Instances (location & Date) for "course1"



Cheers'