2级联动..大类下拉菜单不能控制小类下拉菜单...求解决方法...

<tr>
          <td align="left" valign="middle">&nbsp;</td>
          <td align="right" valign="middle">所属大类别:</td>
          <td align="left" valign="middle"><select name="mite_list" id="select">          
            <%
set rs=Server.CreateObject("Adodb.RecordSet")
zx="Select * from product_list "
rs.Open zx,Conn,1,3
if rs.EOF and rs.BOF then
response.write ("<option selected>暂无大类别<option selected>")
else
Do Until rs.EOF
%>
<option value="<%=rs("id")%>"><%=rs("list")%></option>
<%
rs.MoveNext
loop
end if
%>
          </select>
          </td>
        </tr>
        <tr>
         <td align="left" valign="middle">&nbsp;</td>
          <td align="right" valign="middle">所属小类别:</td>
          <td align="left" valign="middle"><select name="smclass" id="select">
          <%
if list_id<>"" then
set trs=Server.CreateObject("Adodb.RecordSet")
tzx="Select * from class where bgclass_id= " + list_id
trs.open tzx.conn,1,3
if trs.eof and trs.bof then
response.write ("<option selected>暂无小类别<option selected>")
else
do until trs.eof
%>
        <option value="<%=trs("id")%>"><%=trs("smclass")%></option>
        <%
rs.MoveNext
loop
end if
end if
%>
          </select>
          </td>
          </tr>