Update a Select with onchange of other Select

[eluser]Nuno Simões[/eluser]
Thanks, I had some problems getting the selected value, but is solved now.

I have another question, what do I need to load to make the next piece of code to work:

Code:

$.ajax({
      type: "POST",
      url:  "<?php echo site_url("the_url_of_your_ajax_controller")?>",
      data: a,
      success: function(msg){
        $("#ajax_target").html(msg);
        }
    });