radio选中的问题

<table>
<tr>
<td height="27" align="right" bgcolor="#C6EBDE">交易方式：</td>
<td bgcolor="#E1F4EE" id="td1">  <input type="radio" id="fs1" name="fs" value="邮寄">
                 邮寄
            <input type="radio" id="fs2" name="fs" value="自取">
                  自取
                  <input type="radio" id="fs3" name="fs" value="送达" checked>
                  送达
                  <input name="fs" id="fs4" type="radio" value="约定地点" >
                  约定地点</td>
               </tr>
             <tr>
              <td height="27" align="right" bgcolor="#C6EBDE">交易区域：</td>
              <td bgcolor="#E1F4EE">  
                <select name="quyu" id="quyu" >
                  <option value="">选择所在区域</option>
                </select>
             </td>
            </tr>
</table>
<script type="text/javascript">
window.onload = function(){
    var inputs = document.getElementById("td1").getElementsByTagName("input");
    for(var i = 0; i < inputs.length; i++){
        (function(i){
            inputs[i].onclick = function(){
                document.getElementById("quyu").disabled = (i == 2 ? true : false);
            };
        })(i);
    };
};
</script>