select 多選 和單選,分組

<select name="group">
<option value="1">北京</option>
<option value="2">上海</option>
<option value="3">廣州</option>
<option value="4" selected>長沙</option>
</select>
<select multiple="multiple" size="6">
<optgroup label="湖南省">
<option>長沙</option>
<option>株洲</option>
</optgroup>
<optgroup label="湖北省">
<option>武漢</option>
<option>黃岡</option>
<option>咸寧</option>
</optgroup>
</select>
<select multiple="multiple" size="4">
<option>上海</option>
<option>上海2</option>
<option>上海3</option>
<option>上海4</option>
</select>