<select>动态增加<option>的几个问题！！SOS

<html>
<head>
<script language="javascript">
function addItem(){
var opt = document.createElement("OPTION");  
opt.text="4";
opt.value="4";
document.all.sel.add(opt);
}
</script>
</head>
<body >
<form name="frm">
<div id="lb">
<select id="sel">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
</div>
<input type="button" value="ADD" onclick="addItem()">
</form>
</body>
</html>