用我全部的分请教怎么实现这样一个功能

<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title></title></head><body background="img/bg.gif" topmargin="0" marginheight="0"><table width="100%" height="132" border="0" align="center">
  <tr> 
    <td height="46"> 
      <div align="center">
        <p  ><strong><font color="#FF0000">苏州东南区域管理中心车籍资料查询系统</font></strong></p>
      </div></td>
  </tr>
  <tr> 
    <td height="80"> 
      <div align="center">
        <form action="ckmain.asp" method="post" name="form2" target="mainFrame">
          <table width="770" border="0" align="center" cellpadding="0" cellspacing="0" class="table2">
            <tr bgcolor="#99CCFF" class="tbale1"> 
              <td height="28" colspan="6" bgcolor="#CAE7F7" class="tbale1"> <label> 
                </label>
                <div align="center"> 
                  <label> 请选择查询条件 
                  <select name="fhd" id="select" onchange="aa(this)">
                    <option value="1">a</option>
                    <option value="2">b</option>
                  </select>
                  <select id="retor" name="retor" id="retor">
                  </select>
                  <input id="textfield" style="visibility:hidden" type="text" name="textfield">
                  </label>
                </div></td>
            </tr>
            <tr bgcolor="#99CCFF" class="table1"> 
              <td width="183" bgcolor="#CAE7F7" class="tbale1">&nbsp;</td>
              <td colspan="2" align="center" valign="middle" bgcolor="#CAE7F7" class="tbale1"> 
                <input type="submit" name="Submit" value="开始查询"> &nbsp;&nbsp;</td>
              <td align="center" valign="middle" bgcolor="#CAE7F7" class="tbale1">&nbsp;</td>
            </tr>
          </table>
        </form>
      </div></td>
  </tr>
</table>
<script>
function aa(oE)
{
var oA = document.all("retor");
var oB = document.all("textfield");
if(oE.options[oE.selectedIndex].text == 'a')
{
oA.style.visibility = "";
oB.style.visibility = "hidden";
}
else
{
oA.style.visibility = "hidden";
oB.style.visibility = "";
}
}
</script>
</body>
</html>