jQuery基本过滤选择器：
<h1>this is h1</h1>
<div id="p1">
<h2>this is h2</h2>
<input type="text" >
<div id="c1">1</div>
<div id="c2">2
<div id="cc21">21</div>
<div id="cc22">22</div>
<div id="cc23">23</div>
</div>
<div id="c3">3</div>
<div id="c4">4</div>
<div id="c5">5</div>
</div>
