两个select的问题

[xinfang]

父select有四个选择，怎么能让选择第四个选项时把子select给屏蔽掉，就是不时显示出来，选择其它三项时子select又出来，

[xinfang]

gz

[fason]

http://expert.csdn.net/Expert/topic/1505/1505279.xml?temp=.5098993

[zhanghk]

up

[wsj]

<select onchange="document.all.childselect.style.display=(this.selectIndex==3?'none':'block')">
<option value=1>1</option>
<option value=2>2</option>
<option value=3>3</option>
<option value=4>4</option>
</select>
<select id=childselect>
</select>