急救！！！！！！！！！对下拉列表框的值清空，为什么清除不完呢？？？？

其实有简单的方法的
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE> 删除</TITLE>
<META NAME="Generator" CONTENT="EditPlus">
<META NAME="Author" CONTENT="">
<META NAME="Keywords" CONTENT="">
<META NAME="Description" CONTENT="">
<script language="javascript"> 
  
   function qbyidong(optionMenu2)
   {
      
    optionMenu2.options.length=0
   }
</script>
</HEAD><BODY>
    <form action="" name="testform" method="post">
   
      <table width="200" border="1">
        <tr>
          <td rowspan="4"><select name="selectzuo" multiple >
            <option>France</option>
            <option>china</option>
            <option>japan</option>
            <option>english</option>
            <option>Amica</option>
          </select></td>
          <td><input type="button" name="qzy" value="清除选项" onClick="qbyidong(document.testform.selectzuo)"></td>
         
        </tr>
        
      </table>
</form>
</BODY>
</HTML>