为什么在使用一列布局时，会出现重叠
<!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=utf-8" />
<title>信工院主页</title>
<style type="text/css">
body{margin:0;padding:0}
.top{height:100px;}
.main{width:800px;height:600px;margin:0 auto}
.foot{width:800px;height:400px;margin:0 auto}
</style>
</head>
<body>
<div class="top"><img src="img/header.GIF" /></div>
<div class="main"><img src="img/topmiddle.GIF" /></div>
<div class="foot"><img src="img/bottomleft.GIF" /></div>
</body>
</html>
