XML的异常在哪里啊

XML的错误在哪里啊？
header('Content-Type: text/xml ');
echo '';
echo '';
$link=mysqli_connect('localhost','root','root','tbr_121d') 
or die('fail to connect to the database');
$query="SELECT * FROM drill";
$result=mysqli_query($link,$query);
while($row=mysqli_fetch_object($result)) {
echo ''.$row->question.'';
echo ''.$row->option_a.'';
echo ''.$row->option_b.'';
echo ''.$row->option_c.'';
echo ''.$row->option_d.'';
}
echo '';
mysqli_close($link);
?>

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无法显示 XML 页。 
使用 XSL 样式表无法查看 XML 输入。请更正错误然后单击 刷新按钮，或以后重试。 


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XML 文档只能有一个顶层元素。处理资源 'http://localhost/loading_drill' 时出错。第 2 行，位置: 2 

Warning: Cannot modify header information - headers already sent by (output started at C:\AppServ\www\lo...
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