select标签下的选项添加分类

<select multiple="multiple">
 <optgroup label="On-line2">
  <option>A</option>
  <option>B</option>
   <option>C</option>
 </optgroup>
 <optgroup label="On-line2">
  <option>hha</option>
  <option>sa</option>
  <option>ds</option>
 </optgroup>
</select>