js如何实现点击标签文字，文字在文本框出现

<style>.c{ width:40px; height:25px; line-height:25px; text-align:left; margin:2px; float:left; font-size:12px; cursor:pointer }</style>
<input id="i" type="text" ><div class="c" id="c0">市级,</div><div class="c" id="c1">省级,</div><div class="c" id="c2">国家级
<script>
var x=["市级","省级","国家级"];
var y=[];
function $(id){ return document.getElementById(id)};
for(var i=0,m=x.length;i<m;i++){$("c"+i).onclick=(function(i){return function(){
var s=y.join(",").indexOf(x[i]);if(s>=0){
for(var r in y){if(y[r]==x[i]){y.splice(r,1)}}}
else{y.push(x[i])}$("i").value=y.join(" ");}})(i)}
</script>

<style>.label {cursor:pointer}</style>

<input type="text" id="textbox" size = "30" name="chaxinmd" />
<span class="label">JS特效</span>
<span class="label">C++</span>
<span class="label">SQL</span>
<span class="label">狼蚁SEO</span>
<script type="text/javascript">
spans=document.getElementsByTagName("span");
for(i=0;i<spans.length;i++)
{if(spans[i].className=="label")
{  spans[i].onclick=function()
{    document.getElementById('textbox').value=this.innerHTML; 
 }}}
 </script>

<form action="" method="get" name="so_box" id="so_box">

<input name="so_name" type="text" id="so_name" onFocus="if(value==defaultValue){value='';}"
onBlur="if(!value){value=defaultValue;}"
value="请输入的关键词">

<input type="submit" name="Submit" value="提交" onFocus="if(so_box.so_name.value==so_box.so_name.defaultValue){so_box.so_name.value='';}">

</form>