DIV显示和隐藏

<script type="text/javascript">
	function _display(x){
		if($('stat').value == 'b')
			//document.getElementById(x).style.display = "";
			$(x).style.display = "";
		else{
			$(x).style.display = 'none';
		}
	}
	function $(_id){
		return document.getElementById(_id);
	}
</script>

<select  id="stat" name="stat" class="public_text02" onchange = "_display('layer')">
	<option value="">--请选择--</option>
	<option value="a">a</option>
	<option value="b">b</option>
	<option value="c">c</option>
</select>

<div id="layer" style="display:none;">
	When choosing b, the display value of b.
	Otherwise hidden
</div>