Ayuda con ajax

Hola amigos, este código lo saque de un tutorial, pero no se en que parte está mal porque no me funciona.

tuna.html

Código:

<!DOCTYPE html>
<html>
<head>
	<script type ="text/javascript" src ="tuna.js"></script>
</head>
<body onload="process()">
	Here is a list of people...
	<br/>
	<br/>
	<div id="theD"/>
</body>
</html>

tuna.xml

Código:

<?xml version ="1.0" encoding="UTF-8" standalone="yes" ?>
<response>
	<people>
		<person>
			<name>
				Antonio garcia
			</name>
			<ssn>
				sdffggh4456678
			</ssn>
			<name>
				Anid gutierrez
			</name>
			<ssn>
				s34566776783785
			</ssn>
		</person>
	</people>
</response>

tuna.js

Código:

var xmlHttp = createXmlHttpRequestObject();

//create object
function createXmlHttpRequestObject(){
	var xmlHttp;
	if(window.XMLHttpRequest){
		xmlHttp = new XMLHttpRequest();
	}else{
		xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
	}
	return xmlHttp;
}
//called onLoad
function process(){
	if(xmlHttp){
		try{
			xmlHttp.open("GET", "tuna.xml" true);
			xmlHttp.onreadystatechange = handleStateChange;
			xmlHttp.send(null);
		}catch(e){
			alert(e.toString());
		}
	}
}

//when state changes
function handleStateChange(){
	if(xmlHttp.readyState==4){
		if(xmlHttp.status==200){
			try{
				handleResponse();
			}catch(e){
				alert(e.toString());
			}
		}else{
			alert(xmlHttp.statusText);
		}
	}
}

// handle the reponse from the server
function handleResponse(){
	var xmlResponse = xmlHttp.responseXML;
	root = xmlResponse.documentElement;
	names = root.getElementsByTagName("name");
	ssns = root.getElementsByTagName("ssn");
	
	var stuff = "";
	for(var i=0; i < names.length; i++){
		stuff = names.item(i).firstChild.data + " - " + ssns.item(i).firstChild.data + "<br/>";
	}
	
	theD = document.getElementById("theD");
	theD.innerHTML = stuff;
}

El resultado debe ser mas o menos así:

Here is a list of people...

Antonio garcia - sdffggh4456678
Anid gutierrez - s34566776783785