$("ul li:first-child").hide();
$("#showBtn").click(function(){$("ul li:first-child").show();});
$("#hideBtn").click(function(){$("ul li:first-child").hide();});
<h1>Heading<h1>
<ul>
<li>A1</li>
<li>A2</li>
</ul>
<ul>
<li>B1</li>
<li>B2</li>
</ul>
<button id="shouBtn">显示</button>
<button id="hideBtn">隐藏</button>
p {
color: red;
}