<title>可以分类显示的Select下拉框</title>
<select>
<optgroup label="植物">
<option>小麦</option>
<option>玉米</option>
<option>花生</option>
</optgroup>
<optgroup label="动物">
<option>小狗</option>
<option>小猫</option>
<option>猴子</option>
<option>鸡</option>
<option>鸭</option>
</optgroup>
</select>