<head>
<script>
function check(){
var num = document.getElementById("choose").value;
alert(num);
if(num=="4"){
document.getElementById("show").style.display="block";
}else{
document.getElementById("show").style.display="none";
}
}
</script>
</head>
<body onload="check()">
<select id="choose" onchange="check()">
<option value="1">第一</option>
<option value="2">第二</option>
<option value="3">第三</option>
<option value="4">第四</option>
</select>
<div id="show" style="display:none;">
<table>
<tr>
<td>有志者，事竟成</td>
<td>苦心人，天不负</td>
</tr>
</table>
</div>
</body>