onchange jquery ajax

<form name="frm" action="" method="post" onsubmit="return validateSearch(document.frm);" style="margin:0px;">
    <table width="100%" border="0" cellspacing="0" cellpadding="0">
    <tr>
    <td><select name="country" class="textbox" onchange="showActivity(this.options[this.selectedIndex].value,'activity');">
     <option value="" selected="selected">Select Destination</option>
     <option value="1_Nepal">Nepal</option>
     <option value="3_Tibet">Tibet</option>
     <option value="2_Bhutan">Bhutan</option>
     <option value="16_India">India</option>
     <option value="17_Thailand">Thailand</option>
     <option value="18_Sri Lanka">Sri Lanka</option>
     <option value="22_Bangladesh">Bangladesh</option>
   </select></td>
  </tr>
  <tr>
    <td>
  <div id="activity">
  <select name="activity" class="textbox" disabled="disabled">
   <option value="" selected="selected">Select Activity</option>
  </select>
  </div>
  </td>
  </tr>
</form>

I have a following code in my site so when i select the country then i need to options in the activity using the ajax.