为什么自己增加的行可以删除，原来存在的删不掉
<!DOCTYPE html>
<html>
<head>
<title> new document </title>
<meta http-equiv="Content-Type" content="text/html; charset=gbk"/>
<script type="text/javascript">
window.onload = function(){
color()
}
//增加一行
function add(obj){
var tbody = document.getElementById('table');
var tr=document.createElement('tr');
var td=document.createElement('td');
td.innerHTML="<input type='text'>";
tr.appendChild(td);
td=document.createElement('td');
td.innerHTML="<input type='text'>";
tr.appendChild(td);
td=document.createElement('td');
td.innerHTML= "<a href='javascript:;' onclick='deleteRow(this)'>删除</a>";
tr.appendChild(td);
tbody.appendChild(tr);
color();
}
//删除行
function deleteRow(obj){
var tbody = document.getElementById('table');
var tr=obj.parentNode.parentNode;
tbody.removeChild(tr);
}
function color(){
var trs = document.getElementsByTagName('tr');
for(var i =1;i<trs.length;i++){
trs[i].onmouseover = function(){
this.style.backgroundColor ="#f2f2f2";
}
trs[i].onmouseout = function(){
this.style.backgroundColor ="#fff";
}
}
}
</script>
</head>
<body>
<table border="1" width="50%" id="table">
<tr>
<th>学号</th>
<th>姓名</th>
<th>操作</th>
</tr>
<tr>
<td>xh001</td>
<td>王小明</td>
<td><a href="javascript:;" onclick="deleteRow(this)">删除</a></td> <!--在删除按钮上添加点击事件 -->
</tr>
<tr>
<td>xh002</td>
<td>刘小芳</td>
<td><a href="javascript:;" onclick="deleteRow(this)" >删除</a></td> <!--在删除按钮上添加点击事件 -->
</tr>
</table>
<input type="button" value="添加一行" onclick="add()"/> <!--在添加按钮上添加点击事件 -->
</body>
</html>
