JavaScript select 级联操作

  　var s1=    document.getElementById("s1") ;
    var s2=   document.getElementById("s2") ;
    var schoolTerm =["第一学期","第二学期","第三学期","第四学期","第五学期"];
    var content=[
        ["html","css","javascript" ] ,  ["c","c++" ],
        ["mysql","oracle"] ,   ["java" ],
        ["jqury","bootstrap","css3","HTML5"  ]
    ];
    //初始化
   for( var i=0; i<schoolTerm.length ;i++){
       s1.add(new Option( schoolTerm[i],schoolTerm[i])  ,null);
   }
    for( var i=0; i<content[0].length ;i++){
        s2.add(new Option(content[0][i],content[0][i])  ,null);
    }
    /* 选择 */
    s1.onchange = function(){
        var index =   s1.selectedIndex ;
        s2.innerHTML ="";
        for( var i=0; i<content[index].length ;i++){
            s2.add(new Option(content[index][i],content[index][i])  ,null);
        }
    };

<select name="s1" id="s1">

</select>

<select name="s2" id="s2">

</select>