刚学用ASP.NET2008做网页，想调用外部CSS文件，总无法调用，代码如下：
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default4.aspx.cs" Inherits="Default4" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<link rel="stylesheet" href="stylesheet.css" type="text/css" />
<title>三列左右固定宽度中间自适应——AA25.CN</title>
</head>
<body>
<div id="left">左列</div>
<div id="center">中列——AA25.CN</div>
<div id="right">右列</div>
</body>
</html>
外部CSS文件叫stylesheet.css，代码如下：
body {margin:0px;
}
#left {
background-color: #E8F5FE;
border: 1px solid #A9C9E2;
height: 400px;
width: 200px;
position: absolute;
top: 0px;
left: 0px;
}
#center {
background-color: #F2FDDB;
border: 1px solid #A5CF3D;
height: 400px;
margin-right: 202px;
margin-left: 202px;
