Itens / links da lista da barra de navegação do Bootstrap não exibidos

Estou aprendendo a usar o Bootstrap e jogando com componentes, especificamente a barra de navegação. Meus links / itens da lista não estão aparecendo, foram copiados diretamente do Bootstrap. Esperando que alguém possa me explicar o que estou perdendo.

<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
    <meta http-equiv="x-ua-compatible" content="ie=edge">
    <title>NavBar</title>

    <!-- bootstrap css -->
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.2/css/bootstrap.min.css" integrity="sha384-y3tfxAZXuh4HwSYylfB+J125MxIs6mR5FOHamPBG064zB+AFeWH94NdvaCBm8qnd" crossorigin="anonymous">
</head>

<body>


    <nav class="navbar navbar-toggleable-md navbar-light bg-faded">
        <button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbarNavAltMarkup" aria-controls="navbarNavAltMarkup" aria-expanded="false" aria-label="Toggle navigation">
            <span class="navbar-toggler-icon"></span>
        </button>
        <a class="navbar-brand" href="#">Navbar</a>
        <div class="collapse navbar-collapse" id="navbarNavAltMarkup">
            <div class="navbar-nav">
                <a class="nav-item nav-link active" href="#">Home <span class="sr-only">(current)</span></a>
                <a class="nav-item nav-link" href="#">Features</a>
                <a class="nav-item nav-link" href="#">Pricing</a>
                <a class="nav-item nav-link disabled" href="#">Disabled</a>
            </div>
        </div>
    </nav>


    <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.4/jquery.min.js"></script>
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.2/js/bootstrap.min.js" integrity="sha384-vZ2WRJMwsjRMW/8U7i6PWi6AlO1L79snBrmgiDpgIWJ82z8eA5lenwvxbMV1PAh7" crossorigin="anonymous">
    </script>   
</body>

http://codepen.io/hellojessicagraham/pen/yMgXjo

Agradeço antecipadamente

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