Twitter Bootstrap对齐中心。

我很难搞清楚如何把整个事情放在中心位置。我正在使用Twitter Bootstrap 2.Twitter Bootstrap对齐中心。

这是我的代码，我确信我在合适的地方有.container。但是，它们仍然没有调整。

帮助家伙？！

<!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" /> 
<meta http-equiv="X-UA-Compatible" content="IE=edge"> 
<title>Home- Argyle's Automobile</title> 
<link href="bootstrap.css" rel="stylesheet" type="text/css" /> 
</head> 

<body> 
<div class="container banner-header"> 
    <h1><a href="#">Argyle's Automobile</a></h1> 
</div> 

<!--Navbar--> 
<div class="navbar"> 
     <div class="container"> <ul class="nav"> 
       <li class="active"><a href="#">Home</a></li> 
       <li><a href="#">About</a></li> 
       <li><a href="#">Social</a></li> 
      </ul></div> 
</div> 

<!--Marketing Area--> 
<div class="container"><div class="hero-unit"> 

    <h1>Marketing stuff!</h1> 
    <br /> 
    <p>Cras justo odio, dapibus ac facilisis in, egestas eget quam. <br />Fusce dapibus, tellus ac cursus commodo, tortor mauris<br /> condimentum nibh, ut fermentum massa justo sit amet risus.</p> 
    <br /> 
    <br /> 

    <a href="#" class="btn btn-large btn-success">Get Started</a> 
</div></div> 
</div> 

<!--Content--> 
<div id='content' class='container row-fluid'> 
    <div class="span4"> 
     <h3>Content 1</h3> 
     <p>Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat. Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit</p> 
    </div> 
    <div class="span4"> 
     <h3>Content 1</h3> 
     <p>Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat. Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit</p> 
    </div> 
    <div class="span4"> 
     <h3>Content 1</h3> 
     <p>Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat. Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit</p> 
    </div> 
</div> 
</body> 
</html>

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