Jquery $post <select>

Hello i would like to $post a drop down list value. right now i have a auto complete that sends a suggestion back and forth . this is on index.htm

<script type="text/javascript" src="jquery-1.3.2.js"></script>
<script type="text/javascript">
	function lookup(inputString) {
		if(inputString.length == 0) {
			// Hide the suggestion box.
			$('#suggestions').hide();
		} else {
			$.post("rpc.php", {queryString: ""+inputString+""}, function(data){
				if(data.length >0) {
					$('#suggestions').show();
					$('#autoSuggestionsList').html(data);
				}
			});
		}
	} // lookup
	
	
	function fill(thisValue) {
		$('#inputString').val(thisValue);
		setTimeout("$('#suggestions').hide();", 200);
	}

Code (markup):

as you can see in the $ i am sending to rpc.php

if(isset($_POST['queryString'])) {
			$queryString = $db->real_escape_string($_POST['queryString']);

			if(strlen($queryString) >0) {
							
	 $query = $db->query("SELECT s_last, s_city FROM table_student WHERE s_last LIKE '$queryString%' or s_city LIKE '$queryString%'");

Code (markup):

i would like to replace the value in the $query (s_last, s_city) to a value coming from a drop down box. can anyone give me a hand of this plz.