Element类型和HTML元素获取

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8" />
        <title>js_0321作业</title>
        <script src="js/common.js" type="text/javascript" charset="utf-8"></script>
        <style type="text/css">
            
        </style>
    </head>
    <body>

        <img id="myImg" src="img/ft-top.jpg"/>        
        <input type="button" name="" id="btn1" value="第一题a" />
        <input type="button" name="" id="btn2" value="第二题b" />
        <input type="button" name="" id="btn3" value="第三题c" />
        <input type="button" name="" id="btn4" value="第四题d" />
        <input type="button" name="" id="btn5" value="第五题e" />
        <input type="button" name="" id="btn6" value="第六题f" />
        <input type="button" name="" id="btn7" value="第六题g" />
    </body>
</html>
html部分