Bootstrap 4导航栏背景图像未显示

Question

我尝试在Bootstrap 4导航栏中使用背景图像，我觉得很难。我看了几个代码，但没有工作。

我也试过在导航栏之前添加一个div，看看是否会解决它，它仍然没有。

我正在使用Bootstrap 4 Beta 2.我可能做错了什么？

Topnav

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html,
body {
  height: 100%;
}

body {
  -webkit-font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
  color: #6c6f73;
  font-size: 0.9rem;
  background-color: #EEE;
}
.header-feature {

  min-height:400px;
  background: #201D1D url('assets/img/bg/engineer-background.png');
//background: #222 url('assets/img/bg/engineer-background.png') center center no-repeat;
//  background: url('assets/img/bg/engineer-background.png');
  background-size: cover;
  color: #FFF;
  font-size: 0.8rem;
  padding: 0.5rem;

}
.header-feature p {
  font-size: 1rem;
}

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    <div class="header-feature ">
      <nav class="navbar navbar-expand-lg ">
        <a class="navbar-brand" href="#"><img src="assets/img/logo.png" height="50" /></a>
        <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
          <span class="navbar-toggler-icon"></span>
        </button>
        <div class="collapse navbar-collapse" id="navbarNav">
          <ul class="navbar-nav ml-auto">
            <li class="nav-item active">
              <a class="nav-link" href="#"> <span class="sr-only">(current)</span></a>
            </li>
            <li class="nav-item">
              <a class="nav-link" href="#">FOR HR</a>
            </li>
            <li class="nav-item">
              <a class="nav-link" href="#">SIGNUP</a>
            </li>
            <li class="nav-item">
              <a class="nav-link" href="#">LOGIN</a>
            </li>
          </ul>
        </div>
      </nav>

      <div class="header-div">
        <!-- <img src="assets/img/bg/engineer-background.png" /> -->
        <h1>Are you in the groove?</h1>
        <p>
          Share with us
        </p>
        <a  href="#" role="button" class="btn btn-lg btn-success">Get Started</a>
      </div>

    </div>

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