html编程，弹出层遮住下拉框

//脚本

<script language="javascript" type="text/javascript">
 function callObj(){
 var ceng=document.getElementById("filter");
 ceng.style.display="block";
  ceng.style.width=document.body.clientWidth;
        ceng.style.height=document.documentElement.clientHeight;
 }
 function callDiv(){
 var ceng=document.getElementById("filter1");
 ceng.style.display="block";
  ceng.style.width=document.body.clientWidth;
        ceng.style.height=document.documentElement.clientHeight;
 }
 </script>
//内容
<input type="button" onclick="callObj()"  value="遮住下拉框"/>
<input type="button" onclick="callDiv()"  value="遮不住下拉框"/>
<select id="sel" style="z-index:1">
<option value='aa'>aa</option>
</select>
<div id="filter1" style="position: absolute; top: 0px; filter: alpha(opacity=60); background-color: #777;
            z-index: 100; left: 0px; display: none;"></div>
  <OBJECT id="filter"  style="display:none;z-index:   1000;   LEFT:   0px;   WIDTH:   152px;   POSITION:   absolute;   TOP:   5px;   HEIGHT:   200px"   type="text/x-scriptlet"     data="about:这是盖住下拉框的东东"></OBJECT>