Layer hidden par <select> tag

We have a web page with a layer and <select> tags. The layer is a small window which is supposed to hide the <select> but it does not... How can we make it hide the select ?

So it goes :

<style type="text/css">
#cat_prodiv {position:relative;width:15em;}
#cat_proinput {position:relative;width:100%;height:1.2em;display:block;}
#cat_procontainer {position:relative;bottom:4px;right:4px;border:1px solid #404040;background:#fff;font-size:85%;overflow:hidden;}
#cat_proshadow {position:absolute;width:100%;*width:auto;_width:100%;margin:.3em;z-index:5;background:#a0a0a0;} /* bug 614383 workaround for ie7 */
</style>

<div id="cat_prodiv"><input id="cat_proinput" style="height:18px;width:120px;" type="text" name=zoom2 autocomplete=off>
                    <div id="cat_proshadow"><div id="cat_procontainer">bla bla bla<br>bla bla bla<br>bla bla bla<br>
                        bla bla bla<br>bla bla bla<br>bla bla bla<br>bla bla bla<br></div></div>
                      </div>
<br><br>
<select><option>hello</select>

and we want to "hello" to be hidden by the "bla bla bla" lines.

Help greatly appreciated.