让DIV层显示在select控件上面的方法

<form id="Form1" method="post" runat="server">
<select>
<option value="">11</option>
<option value="">22</option>
<option value="">33</option>
</select>
<div id="menu" style="z-index:100; left:20px; width:100px; position:absolute; top:20px; height:200px; background-color:#6699cc">
<table>
<tr>
<td>item 1</td>
</tr>
<tr>
<td>item 2</td>
</tr>
<tr>
<td>item 3</td>
</tr>
<tr>
<td>item 4</td>
</tr>
<tr>
<td>item 5</td>
</tr>
</table>
<iframe src="/javascript:false" style="z-index:-1; filter:progid:DXImageTransform.Microsoft.Alpha(style=0,opacity=0); left:0px; visibility:inherit; width:100px; position:absolute; top:0px; height:200px"> </iframe>
</div>
</form>