为什么alert 两遍啊？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script type="text/javascript" src="https://www.imooc.com/static/lib/jquery/1.9.1/jquery.js"></script>
<title></title>
</head>
<body>
<script >alert($) </script>
</body>
</html>
