Bootstrap 4导航栏分为两行

所有。我是Web开发的新手，正在研究它。 当我尝试制作导航栏并从教程中复制代码时，外观并不相同。即使导航栏品牌旁边有足够的空间，所有导航项也会在另一行中列出。您能帮我找出原因吗？ 这是我的代码和屏幕截图：

<nav class="navabar navbar-expand-md navbar-light fixed-top" style="background-color: #e3f2fd">
    <a href="#" class="navbar-brand">Brand</a>

    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
            <span class="navbar-toggler-icon"></span>
    </button>

    <div class="collapse navbar-collapse" id="navbarSupportedContent">
        <ul class="navbar-nav mr-auto">
            <li class="nav-item active">
                <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Link</a>
            </li>
            <li class="nav-item">
                <a class="nav-link disabled" href="#">Disabled</a>
            </li>
        </ul>
        <form class="form-inline my-2 my-lg-0">
            <input class="form-control mr-sm-2" type="text" placeholder="Search" aria-label="Search">
            <button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
        </form>
    </div>
</nav>

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