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為什么40行刪除函數不能刪除本來就有的兩個tr，用46，47行卻都能刪除?

<!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(){???????????
?????//?鼠標移動改變背景,可以通過給每行綁定鼠標移上事件和鼠標移除事件來改變所在行背景色。
?????var?trc?=?document.getElementsByTagName("tr");
????????
????????for(var?i=1;i<trc.length;i++){
????????????trc[i].onmouseover=function?(){this.style.backgroundColor="#f2f2f2";}
????????????trc[i].onmouseout=function?(){this.style.backgroundColor="#fff";}
????????}
?????}
?????
??????//?編寫一個函數，供添加按鈕調用，動態在表格的最后一行添加子節點；
?????function?createc(){
?????????var?tablec?=?document.getElementById("table");
?????????var?newtr?=?document.createElement("tr");

?????????var?newtd?=?document.createElement("td");
?????????newtd.innerHTML?=?"<input?type='text'/>";
?????????newtr.appendChild(newtd);

?????????var?newtd?=?document.createElement("td");
?????????newtd.innerHTML?=?"<input?type='text'/>";
?????????newtr.appendChild(newtd);

?????????var?newtd?=?document.createElement("td");
?????????newtd.innerHTML?=?"<a?href='javascript:;'?onclick='removetr(this)'>刪除</a>";
?????????newtr.appendChild(newtd);

?????????tablec.appendChild(newtr);
?????}
???
????????
?????//?創建刪除函數
?????function?removetr(obj){
?????????
???????????var?tablec?=?document.getElementById("table");
???????????var?tr?=?obj.parentNode.parentNode;
??????????tablec.removeChild(tr);?
????????
?????//????var?tr?=?obj.parentNode.parentNode;
?????//????tr.parentNode.removeChild(tr);
?????}

??</script>?
?</head>?
?<body>?
???????<table?border="1"?width="50%"?id="table">
???????<tr>
????????<th>學號</th>
????????<th>姓名</th>
????????<th>操作</th>
???????</tr>??

???????<tr>
????????<td>xh001</td>
????????<td>王小明</td>
????????<td><a?href="javascript:;"?onclick="removetr(this)">刪除</a></td>???<!--在刪除按鈕上添加點擊事件??-->
???????</tr>

???????<tr>
????????<td>xh002</td>
????????<td>劉小芳</td>
????????<td><a?href="javascript:;"?onclick="removetr(this)">刪除</a></td>???<!--在刪除按鈕上添加點擊事件??-->
???????</tr>??

???????</table>
???????<input?type="button"?value="添加一行"?onclick="createc()"?/>???<!--在添加按鈕上添加點擊事件??-->
????
?</body>
</html>