jq问题

<div id="box">
		<p>
			<span>A</span>
			<span>B</span>
		</p>
		<div id="box2">
			<span id="span3">C</span>
			<span id="span4">D</span>
			<span>E</span>
		</div>
</div>

　　弹出

<span id="span4">D</span>在所有span中的所因为3

alert($("#box span").index($('#span4')));

　　弹出

<span id="span3">C</span>在box2中的索引为0

alert($("#box2 span").index($("#span3")));