用javascript实现不按Ctrl实现Multiple Select多选_表单特效

<meta http-equiv="Content-Type" content="text/html;charset=UTF-8"> <title> New Document </title> <meta name="Generator" content="EditPlus"> <meta name="Author" content=""> <meta name="Keywords" content=""> <meta name="Description" content=""> <select id="oselect" name="cars" size="10" multiple><option value="BMW">宝马</option> <option value="Porsche">保时捷</option> <option value="Benz">奔驰</option> <option value="Santana">桑塔纳</option> <option value="LK">林肯</option> <option value="?">标志</option> <option value="Bus">大公共</option></select><input type="button" id="btnShowSelected" value="查看选中项索引">
[Ctrl+A 全选 注:如需引入外部Js需刷新才能执行]