如何增加标签

<ul id="a">     <li>aaaaaaaaa</li>     <li>bbbbbbbbb</li>     <li>ccccccccc</li> </ul> <br/> <input type="button" value="增加10节点" onclick="c()"/> <br/><br/><br/><br/> <!--第二题--> <div class="left">     <select id="lstLeft" size="8"></select> </div> <div class="middle">     <input type="button" id="add" value=">"/>     <input type="button" id="addAll" value=">>"/>     <input type="button" id="remove" value="<"/>     <input type="button" id="removeAll" value="<<"/> </div> <div class="right">     <select id="lstRight" size="8"></select> </div> <br/> <form action="#" method="post" id="frmTest" style="clear: both">     <select  id="lstNews" size="8" multiple="multiple" style="display: block">         <option value="1">aaaaa</option>         <option value="2">bbbbb</option>         <option value="3">ccccc</option>         <option value="4">ddddd</option>         <option value="5">eeeee</option>     </select>     <input type="button" id="btnUp" value="向上"/>     <input type="button" id="btnDown" value="向下"/>     <input type="button" id="btnDelete" value="删除"/> </form> </body> <script type="text/javascript">     var str=[];     var a=document.getElementById("a");     function c(){         for(var i=1;i<=10;i++){             var b=document.createElement("li");             var b1=document.createTextNode(i+"增加标签");             str[i-1]=b.appendChild(b1); //            a.appendChild(str[i-1]);         }         for(var w=0;w<str.length;w++){            var c=document.createElement("div");             var c1=document.createTextNode(str[w]);             c.appendChild(c1);             a.appendChild(c)         }     }