bootstrap的js文件引用只有放在最开头才能被使用，放在底部的话bootstrap的样式还有jQuery库都不能使用
<script src="//cdn.bootcss.com/respond.js/1.4.2/respond.js" type="text/javascript" ></script>
<script src="https://oss.maxcdn.com/respond/1.4.2/respond.min.js" type="text/javascript"></script>
<script src="jquery-3.1.1.min.js" type="text/javascript" charset="utf-8"></script>
<script src="bootstrap-3.3.7-dist/js/bootstrap.min.js" type="text/javascript" charset="utf-8"></script>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width,initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="IE=edge,Chrome=1" />
<title>Title</title>
<link rel="stylesheet" href="bootstrap-3.3.7-dist/css/bootstrap.min.css" media="screen">
</head>
