javascript下拉框不被选中大类的区别方法

<meta http-equiv="content-type" content="text/html; charset=utf-8"> <title>runcode</title> <meta name="author" content="sheneyan"> <select><optgroup label="a"> <option>a1 </option> <option>a2 </option> <option>a3 </option> </optgroup> <optgroup label="b"> <option>b1 </option> <option>b2 </option> <option>b3 </option> </optgroup></select>
[ctrl+a 全选 注:如需引入外部js需刷新才能执行]