css ul li导航菜单居中问题解决方法

<ul id="nav">
<li><a href="#">home</li>
<li><a href="#">advice</li>
<li><a href="#">page</li>
<li><a href="#">people</li>
<li><a href="#">service</li>
</ul>

<style type="text/css">
{margin:0; padding:0;}
ul,li{list-style:none;}
.clear{clear:both;}
#nav{width:100%; white-space:nowrap; overflow:hidden; background:#c; position:relative; }
#nav li{ float:left; position:relative; left:50%; }
#nav .a1{margin-left:-240px;} #nav .a2{margin-left:-160px;} #nav .a3{margin-left:-80px;}
#nav li a{ display:block; text-align:center; line-height:28px; background:#c; color:#000; width:80px; height:28px;}
</style>
<ul id="nav">
<li class="a1"><a href="#">home</a></li>
<li class="a2"><a href="#">advice</a></li>
<li class="a3"><a href="#">page</a></li>
<li><a href="#">people</a></li>
<li><a href="#">service</a></li>
</ul>