用我全部的分请教怎么实现这样一个功能
当我选择fhd下拉列表的a时显示retor列表框,选b时显示文本框,如何实现
<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">
<option value="1">a</option>
<option value="2">b</option>
</select>
<select name="retor" id="retor">
</select>
<input type="text" name="textfield">
</label>
</div></td>
</tr>
<tr bgcolor="#99CCFF" class="table1">
<td width="183" bgcolor="#CAE7F7" class="tbale1"> </td>
<td colspan="2" align="center" valign="middle" bgcolor="#CAE7F7" class="tbale1">
<input type="submit" name="Submit" value="开始查询"> </td>
<td align="center" valign="middle" bgcolor="#CAE7F7" class="tbale1"> </td>
</tr>
</table>
</form>
</div></td>
</tr>
</table>
</body>
</html>