夺命雷公狗jquery---7子元素选择器

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title></title>
        <script src="js/jquery.js"></script>
        <script>
            window.onload = function(){
                document.getElementById('btnok').onclick = function(){
                    //1.匹配索引为3的子元素
                    //$('li:nth-child(3)').html('test');
                    //2.匹配第一个子元素
                    //$('li:first-child').html('test');
                    //3.匹配值具有一个子元素的元素
                    $('li:only-child').html('test');
                }
            }
        </script>
    </head>
    <body>
        <input type="button" id="btnok" value="匹配" />
        <hr>
        <ol>
            <li>AAAAA</li>
            <li>BBBBB</li>
            <li>CCCCC</li>
        </ol>
        <hr>
        <ol>
            <li>DDDDD</li>
        </ol>
    </body>
</html>