Twitter bootstrap 3 嵌入的响应youtube视频未显示

我正在使用bootstrap，我想将一些视频从youtube添加到我的网站。我看过一些这样做的代码，但当我尝试它不起作用，视频没有显示

代码如下：

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap.min.css">

<!-- jQuery library -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>

<!-- Latest compiled JavaScript -->
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/js/bootstrap.min.js"></script>

<!-- 16:9 aspect ratio -->
<div class="embed-responsive embed-responsive-16by9">
  <iframe class="embed-responsive-item" src="https://www.youtube.com/watch?v=PIh2xe4jnpk"></iframe>
</div>

<!-- 4:3 aspect ratio -->
<div class="embed-responsive embed-responsive-4by3">
  <iframe class="embed-responsive-item" src="https://www.youtube.com/watch?v=PIh2xe4jnpk"></iframe>
</div>   

---

您输入了错误的yotube URL。你需要嵌入视频

<!-- 16:9 aspect ratio -->
<div class="embed-responsive embed-responsive-16by9">
  <iframe class="embed-responsive-item" src="https://www.youtube.com/embed/PIh2xe4jnpk"></iframe>
</div>
<!-- 4:3 aspect ratio -->
<div class="embed-responsive embed-responsive-4by3">
  <iframe class="embed-responsive-item" src="https://www.youtube.com/embed/PIh2xe4jnpk"></iframe>
</div> 

---

您输入了错误的yotube URL。你需要嵌入视频

<!-- 16:9 aspect ratio -->
<div class="embed-responsive embed-responsive-16by9">
  <iframe class="embed-responsive-item" src="https://www.youtube.com/embed/PIh2xe4jnpk"></iframe>
</div>
<!-- 4:3 aspect ratio -->
<div class="embed-responsive embed-responsive-4by3">
  <iframe class="embed-responsive-item" src="https://www.youtube.com/embed/PIh2xe4jnpk"></iframe>
</div> 

哦，天哪，你说得对，我没有注意到哈哈哈，非常感谢！哦，天哪，你说得对，我没注意到哈哈哈，非常感谢！