我的上一页，下一页的箭头不显示是什么原因呢
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<!-- 上述3个meta标签*必须*放在最前面，任何其他内容都*必须*跟随其后！ -->
<title>Bootstrap 101 Template</title>
<!-- Bootstrap -->
<link href="css/bootstrap.min.css" rel="stylesheet">
<!-- HTML5 shim and Respond.js for IE8 support of HTML5 elements and media queries -->
<!-- WARNING: Respond.js doesn't work if you view the page via file:// -->
<!--[if lt IE 9]>
<script src="//cdn.bootcss.com/html5shiv/3.7.2/html5shiv.min.js"></script>
<script src="//cdn.bootcss.com/respond.js/1.4.2/respond.min.js"></script>
<![endif]-->
</head>
<style>
body{ padding-top:50px;}
.carousel{ height:500px; margin-bottom:60px;}
.carousel .item{ height:500px;}
.carousel .item img{ width:100%;}
/* 响应式布局 */
@media (max-width: 768px) {
.summary {
padding-right: 3px;
padding-left: 3px;
}
.carousel {
height: 300px;
margin-bottom: 30px;
}
.carousel .item {
height: 300px;
}
.carousel img {
min-height: 300px;
}
.carousel-caption p {
font-size: 16px;
line-height: 1.4;
}
.feature-heading {
font-size: 34px;
}
.feature-heading .text-muted {
font-size: 22px;
}
}
@media (min-width: 992px) {
.feature-heading {
margin-top: 120px;
}
} /* 响应式布局 */
@media (max-width: 768px) {
.summary {
padding-right: 3px;
padding-left: 3px;
}
.carousel {
height: 300px;
margin-bottom: 30px;
}
.carousel .item {
height: 300px;
background-color: #000;
}
.carousel img {
min-height: 300px;
}
.carousel-caption p {
font-size: 16px;
line-height: 1.4;
}
.feature-heading {
font-size: 34px;
}
.feature-heading .text-muted {
font-size: 22px;
}
}
@media (min-width: 992px) {
.feature-heading {
margin-top: 120px;
}
}
</style>
<body>
<nav class="navbar navbar-inverse navbar-fixed-top">
<div>
<!-- Brand and toggle get grouped for better mobile display -->
<div>
<button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1" aria-expanded="false">
<span>Toggle navigation</span>
<span></span>
<span></span>
<span></span>
</button>
<a href="#">现代浏览器博物管</a>
</div>
<!-- Collect the nav links, forms, and other content for toggling -->
<div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
<ul class="nav navbar-nav">
<li><a href="#">综述 <span>(current)</span></a></li>
<li><a href="#">简述</a></li>
<li>
<a href="#" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">特点 <span></span></a>
<ul>
<li><a href="#">Action</a></li>
<li><a href="#">Another action</a></li>
<li><a href="#">Something else here</a></li>
<li role="separator"></li>
<li><a href="#">Separated link</a></li>
<li role="separator"></li>
<li><a href="#">One more separated link</a></li>
</ul>
</li>
<li><a href="#" title="" data-toggle="modal" data-target="#about-modal">关于</a></li>
</ul>
</div><!-- /.navbar-collapse -->
</div><!-- /.container-fluid -->
</nav>
<!--轮播-->
<div id="ad-carousel" class="carousel slide" data-ride="carousel">
<ol>
<li data-target="#ad-carousel" data-slide-to="0"></li>
<li data-target="#ad-carousel" data-slide-to="1"></li>
<li data-target="#ad-carousel" data-slide-to="2"></li>
<li data-target="#ad-carousel" data-slide-to="3"></li>
<li data-target="#ad-carousel" data-slide-to="4"></li>
</ol>
<div>
<div class="item active">
<img src="images/chrome-big.jpg" alt="1 slide">
<div>
<div>
<h1>Chrome</h1>
<p>Google Chrome，又称Google浏览器，是一个由Google（谷歌）公司开发的网页浏览器。</p>
<p><a class="btn btn-lg btn-primary" href="http://www.google.cn/intl/zh-CN/chrome/browser/"
role="button" target="_blank">点我下载</a></p>
</div>
</div>
</div>
<div>
<img src="images/chrome-big.jpg" alt="2 slide">
<div>
<div>
<h1>Firefox</h1>
<p>Mozilla Firefox，中文名通常称为“火狐”或“火狐浏览器”，是一个开源网页浏览器。</p>
<p><a class="btn btn-lg btn-primary" href="http://www.firefox.com.cn/download/" target="_blank"
role="button">点我下载</a></p>
</div>
</div>
</div>
<div>
<img src="images/safari-big.jpg" alt="3 slide">
<div>
<div>
<h1>Safari</h1>
<p>Safari，是苹果计算机的最新操作系统Mac OS X中的浏览器。</p>
<p><a class="btn btn-lg btn-primary" href="http://www.apple.com/cn/safari/" target="_blank"
role="button">点我下载</a></p>
</div>
</div>
</div>
<div>
<img src="images/opera-big.jpg" alt="4 slide">
<div>
<div>
<h1>Opera</h1>
<p>Opera浏览器，是一款挪威Opera Software ASA公司制作的支持多页面标签式浏览的网络浏览器。</p>
<p><a class="btn btn-lg btn-primary" href="http://www.opera.com/zh-cn" target="_blank"
role="button">点我下载</a></p>
</div>
</div>
</div>
<div>
<img src="images/ie-big.jpg" alt="5 slide">
<div>
<div>
<h1>IE</h1>
<p>Internet Explorer，简称 IE，是微软公司推出的一款网页浏览器。</p>
<p><a class="btn btn-lg btn-primary" href="http://ie.microsoft.com/" target="_blank"
role="button">点我下载</a></p>
</div>
</div>
</div>
</div>
<a class="left carousel-control" href="#ad-carousel" data-slide="prev">
<span class="glyphicon glyphicon-chevron-left"></span>
</a>
<a class="right carousel-control" href="#ad-carousel" data-slide="next">
<span class="glyphicon glyphicon-chevron-right"></span>
</a>
</div>
<!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
<script src="js/jquery-1.11.1.min.js"></script>
<!-- Include all compiled plugins (below), or include individual files as needed -->
<script src="js/bootstrap.min.js"></script>
</body>
</html>
