用javascript动态更改DropDownList下拉项

  function clearitem(){ var drp1 = document.getElementById("drp1");        while(drp1.options.length>0)        {           drp1.options.remove(0);        } }//动态更改方法（根据城市代码取得该市商业区并添加到DropDownList中） function getsyq()    {        var city = document.getElementById("DropDownList_Cities").value;　　//取得城市代码        var htp = new ActiveXObject("Msxml2.XMLHTTP");        var drp1 = document.getElementById("drp1");　        var url = "?stat=1&city="+city　　        htp.open("post",url,true)        htp.onread