怎么获取选择下拉框，添加到页面上表格中的的值？


<!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="1">1</option>
        <option value="2">2</option>
    </select>
    <table id="tab">
        <tr>
            <td>1</td>
        </tr>
    </table>
    <input type="button" value="添加" id="btn" />
    <script>
        document.getElementById('btn').onclick = function(){
            var value = document.getElementById('sel')[document.getElementById('sel').selectedIndex].value;
            var tr = document.createElement('tr');
            var td = document.createElement('td');
            td.innerHTML = value;
            tr.appendChild(td);
            document.getElementById('tab').appendChild(tr);
        };
    </script>
</body>
</html>