用javascript实现不按Ctrl实现Multiple Select多选

<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]