<html>
<head>
<script type="text/javascript">
function action()
{
document.getElementById('box').options.length = 0;
document.getElementById('box').options.add(new Option('a','a'));
document.getElementById('box').options.add(new Option('b','b'));
document.getElementById('box').options.add(new Option('c','c'));
document.getElementById('box').selectedIndex = -1;
}
</script>
</head>
<body onload="action();">
<select size="1" id="box" onchange="alert(this.value);"></select>
</body>
</html>