表单控件中optgroup标签的操作实例是什么呢？

<button id="btn1a" type="button">启用</button>
<button id="btn1b" type="button">禁用</button>
<button id="btn2a" type="button">可多选</button>
<button id="btn2b" type="button">不可多选</button>
<button id="btn3a" type="button">size=1</button>
<button id="btn3b" type="button">size=2</button>
<button id="btn3c" type="button">size=3</button>
<button id="btn3d" type="button">不设置size</button>    
<form action="#">
    <br><br>
    <select name="test" id="select"> 
        <optgroup label="num">
            <option value="11" disabled>1</option>
            <option value="22" selected>2</option>
            <option value="33">3</option>
            <option value="44" label="word">4</option>        
        </optgroup>
        <optgroup  label="word">
            <option>a</option>
            <option>b</option>
            <option>c</option>
            <option>d</option>        
        </optgroup>
        <optgroup  label="汉字" disabled>
            <option value="一个">一</option>
            <option value="二个">二</option>
            <option value="三个">三</option>
            <option value="四个">四</option>        
        </optgroup>    
    </select>
    <input type="submit">    
</form>
<script>
    var select = document.getElementById('select');
    btn1a.onclick = function(){
        select.removeAttribute('disabled');
    }    
    btn1b.onclick = function(){
        select.setAttribute('disabled','');
    }
    btn2a.onclick = function(){
        select.setAttribute('multiple','');
    }
    btn2b.onclick = function(){
        select.removeAttribute('multiple');
    }
    btn3a.onclick = function(){
        select.setAttribute('size','1');
    }
    btn3b.onclick = function(){
        select.setAttribute('size','2');
    }
    btn3c.onclick = function(){
        select.setAttribute('size','3');
    }
    btn3d.onclick = function(){
        select.removeAttribute('size');
    }
</script>