请问为什么通过XMLHttpRequest无法正常返回json？

xmlHttp=null;
function GetJS()
{
try
{// Firefox, Opera 8.0+, Safari, IE7
xmlHttp=new XMLHttpRequest();
}
catch(e)
{// Old IE
try
{
xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
}
catch(e)
{
alert ("Your browser does not support XMLHTTP!");
return;
}
}
url="http://api.map.baidu.com/place/v2/search?&query=公交站&region=北京&scope=2&page_size=20&output=kml&ak=E4805d16520de693a3fe707cdc962045";
xmlHttp.open("GET",url,true);
xmlHttp.send(null);
alert(xmlHttp.responseText);
};
GetJS();