html输入框问题

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
</head><body>
<select id="sel">
<option value="a.com">a.com</option>
    <option value="b.com">b.com</option>
</select>
<input type="text" id="inp" />
<input type="button" value="submit" id="sub" />
<script type="text/javascript">
document.getElementById("sub").onclick=function(){
window.open('http://'+document.getElementById('sel').value+'/'+document.getElementById('inp').value,'test')
}
</script>
</body>
</html>