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為什么要寫成getElementByTagName("li")[2]?

<!DOCTYPE?HTML>
<html>
<head>
<meta?http-equiv="Content-Type"?content="text/html;?charset=utf-8">
<title>nextSibling</title>
</head>
<body>
<ul?id="u1">???
????????????<li?id="a">javascript</li>???
????????????<li?id="b">jquery</li>???
????????????<li?id="c">html</li>???
????????</ul>???
????????<ul?id="u2">???
????????????<li?id="d">css3</li>???
????????????<li?id="e">php</li>???
????????????<li?id="f">java</li>???
????????</ul>???
<script?type="text/javascript">
????function?get_nextSibling(n){
????????var?x=n.nextSibling;
????????while?(x?&&?x.nodeType!=1){
????????????x=x.nextSibling;
????????}
????????return?x;
????}

????var?x=document.getElementsByTagName("li")[0];
????document.write(x.nodeName);
????document.write("?=?");
????document.write(x.innerHTML);
????
????var?y=get_nextSibling(x);
????
????if(y!=null){
????????document.write("<br?/>nextsibling:?");
????????document.write(y.nodeName);
????????document.write("?=?");
????????document.write(y.innerHTML);
????}else{
??????document.write("<br>已經(jīng)是最后一個節(jié)點");??????
????}
function?get_previousSibling(n){
????????var?a=n.previousSibling;
????????while(a&&a.nodeType!=1){
????????????a=a.previousSibing;
????????}
????}
????
?????var?a=document.getElementsByTagName("li")[2];
????document.write("<br/>"+a.nodeName);
????document.write("=");
????document.write(a.innerHTML);
????
????var?b=get_previousSibling(a);

????if(b!=null){?
????????document.write("<br/>previoussibling:");
????????document.write(b.nodeName);
????????document.write("=");
????????document.write(b.innerHTML);
????}
????else{???
????????document.write("<br/>已經(jīng)是第一個節(jié)點");
????}
</script>
</body>
</html>

第49行，為什么下標要寫成2？我疑惑的是2是怎么來的？