页面第一次刷新,默认北京市,点击北京市无法显示市区SELECT的数值,点击其他省再切回来就可以

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
<title>Document</title>
    <script type="text/javascript" src="index.js"></script>
</head>
<body onload="address()">
<form>
省份：<select name="sheng" id="sheng" onchange="linkage()"></select>
<!--  请在此补充代码 -->
市/区：<select name="shiqu" id="shiqu"></select>
<!--  请在此补充代码 -->
</form>
</body>
</html>





var arrsh=new Array("北京市","天津市","河北省","河南省");
var arrshiqu=new Array("朝阳区","西城区","西青区","武清区","石家庄","邯郸市","郑州市","开封市");
function address(){
     /*请在此补充代码*/ 
     var sheng=document.getElementById("sheng");
     var shiqu=document.getElementById("shiqu");
    
     for(var i=0;i<arrsh.length;++i)
     {
         selectAdd(sheng,arrsh[i],arrsh[i]);
     }
}
function selectAdd(obj,name,value){
/*请在此补充代码*/  
obj.options.add(new Option(name,value));
}

function linkage(){
/*请在此补充代码*/ 
var sheng=document.getElementById("sheng");
    var shiqu=document.getElementById("shiqu");
    shiqu.options.length=0;//删除下拉框的值
    var firstvalue=sheng.value;//获取下拉框选中的值
    switch(firstvalue){
        case "北京市":
            selectAdd(shiqu,arrshiqu[0],arrshiqu[0]);
            selectAdd(shiqu,arrshiqu[1],arrshiqu[1]);
            break;
        case "天津市":
            selectAdd(shiqu,arrshiqu[2],arrshiqu[2]);
            selectAdd(shiqu,arrshiqu[3],arrshiqu[3]);
            break;
        case "河北省":
            selectAdd(shiqu,arrshiqu[4],arrshiqu[4]);
            selectAdd(shiqu,arrshiqu[5],arrshiqu[5]);
            break;
        case "河南省":
            selectAdd(shiqu,arrshiqu[6],arrshiqu[6]);
            selectAdd(shiqu,arrshiqu[7],arrshiqu[7]);
            break;
    }
}