用javascript应用一例
举个例子，怎么用自己看着办！
<html>
<head>
<title>下拉菜单</title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<script language="javascript">
function chselect()
{
    var value;
    value=document.insert.serv.value;
    if(value=="0")
    {
        ask1.style.display='none';

    }
    else if(value=="1")
    {
        ask1.style.display='';
        insert.ask.value="呵呵，看来你很开心嘛！";
    }
    else if(value=='2'){
        ask1.style.display='';
        insert.ask.value="怎么啦，不开心？";
    }
}
</script>
</head>

<body bgcolor="#FFFFFF">
<form name="insert" method="post" action="" >
 <table width="550" border="1" cellspacing="0" cellpadding="3" bordercolorlight="#666666" bordercolordark="#B0CAFF">
  <tr>
   <td align="center" class="textm" bgcolor="#FFFFCC"> 你好吗？ </td>
   <td class="texts" height="32" bgcolor="#FFFFCC">
    <select name="serv" onChange=chselect() class="area">
     <option value="0" selected></option>
     <option value="1">好的很</option>
     <option value="2">一点都不好</option>
    </select>
   </td>
   <td id=ask1 style="DISPLAY:none" class="texts" bgcolor="#CCFFCC">
    <input type="text" name="ask" size="20" class="area">
   </td>
  </tr>
 </table>
</form>
</body>
</html>