jquery实现下拉框功能

说不清楚，直接上图

<!DOCTYPE html>

<html lang="en" xmlns="http://www.w3.org/1999/xhtml">
<head>
    <meta charset="utf-8" />
    <title></title>
    <style type="text/css">
        * {
            margin: 0;
            padding: 0;
        }

        div.centent {
            float: left;
            text-align: center;
            margin: 10px;
        }

        span {
            display: block;
            margin: 2px 2px;
            padding: 4px 10px;
            background: #898989;
            cursor: pointer;
            font-size: 12px;
            color: white;
        }
    </style>
    <script src="jquery-1.3.2.min.js"></script>
    <script>
        $(function () {
            $("#add").click(function () {//单个添加
                var $options = $("#select1 option:selected");
                $options.appendTo("#select2");
            });
            $("#add_all").click(function () {//全部添加
                $("#select1 option").each(function () {
                    $(this).appendTo("#select2");
                    /*
                    也可以写为：
                    var p=$("#select1 option");
                    p.appendTo("#select2");
                    */
                });
            });
            $("#remove").click(function () {//同上，只不过方向相反
                var $options = $("#select2 option:selected");
               // var $remove = $options.remove();
                $options.appendTo("#select1");
            });
            $("#remove_all").click(function () {//同上，只不过方向相反
                var p = $("#select2 option");
                p.appendTo("#select1");
            });
        });
       
    </script>
</head>
<body>
    <div class="centent">
        <select multiple id="select1" style="width:100px;height:160px">
            <option value="1">选项1</option>
            <option value="2">选项2</option>
            <option value="3">选项3</option>
            <option value="4">选项4</option>
            <option value="5">选项5</option>
            <option value="6">选项6</option>
        </select>

        <div>
            <span id="add">选中添加到右边&gt;&gt;</span>
            <span id="add_all">全部添加到右边&gt;;&gt;</span>
        </div>
    </div>
    <div class="centent">
        <select multiple id="select2" style="width:100px;height:160px"></select>

        <div>
            <span id="remove">&lt;&lt;选中删除到左边</span>
            <span id="remove_all">&lt;&lt;全部删除到左边</span>
        </div>
    </div>
</body>
</html>