Your #nav-bar should always be at the top of the viewport, ya trate de hacer de todo y no pude solucionar

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    <div id="page-wrapper">

      <header id="header">

        <div class="logo">

          <img id="header-img" src="https://cdn.freecodecamp.org/testable-projects-fcc/images/product-landing-page-logo.png" alt="original trombones logo">

        </div>

        <nav id="nav-bar">

          <ul>

            <li><a class="nav-link" href="#features">Features</a></li>

            <li><a class="nav-link" href="#how-it-works">How It Works</a></li>

            <li><a class="nav-link" href="#pricing">Pricing</a></li>

          </ul>

        </nav>

      </header>

      <div class="container"></div>

      <section id="hero">

        <h2>Handcrafted, home-made masterpieces</h2>

        <form id="form" action="https://www.freecodecamp.com/email-submit">

          <input name="email" id="email" type="email" placeholder="Enter your email address" required="">

          <input id="submit" type="submit" value="Get Started" class="btn">

        </form>

      </section>

      <div class="container">

        <section id="features">

          <div class="grid">

            <div class="icon"><i class="fa fa-3x fa-fire"></i></div>

            <div class="desc">

              <h2>Premium Materials</h2>

              <p>

                Our trombones use the shiniest brass which is sourced locally.

                This will increase the longevity of your purchase.

              </p>

            </div>

          </div>

          <div class="grid">

            <div class="icon"><i class="fa fa-3x fa-truck"></i></div>

            <div class="desc">

              <h2>Fast Shipping</h2>

              <p>

                We make sure you recieve your trombone as soon as we have

                finished making it. We also provide free returns if you are not

                satisfied.

              </p>

            </div>

          </div>

          <div class="grid">

            <div class="icon">

              <i class="fa fa-3x fa-battery-full" aria-hidden="true"></i>

            </div>

            <div class="desc">

              <h2>Quality Assurance</h2>

              <p>

                For every purchase you make, we will ensure there are no damages

                or faults and we will check and test the pitch of your

                instrument.

              </p>

            </div>

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        <section id="how-it-works">

          <iframe id="video" height="315" src="https://www.youtube-nocookie.com/embed/y8Yv4pnO7qc?rel=0&amp;controls=0&amp;showinfo=0" frameborder="0" allowfullscreen=""></iframe>

        </section>

        <section id="pricing">

          <div class="product" id="tenor">

            <div class="level">Tenor Trombone</div>

            <h2>$600</h2>

            <ol>

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            </ol>

            <button class="btn">Select</button>

          </div>

          <div class="product" id="bass">

            <div class="level">Bass Trombone</div>

            <h2>$900</h2>

            <ol>

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            </ol>

            <button class="btn">Select</button>

          </div>

          <div class="product" id="valve">

            <div class="level">Valve Trombone</div>

            <h2>$1200</h2>

            <ol>

              <li>Plays similar to a Trumpet</li>

              <li>Great for Jazz Bands</li>

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            </ol>

            <button class="btn">Select</button>

          </div>

        </section>

        <footer>

          <ul>

            <li><a href="#">Privacy</a></li>

            <li><a href="#">Terms</a></li>

            <li><a href="#">Contact</a></li>

          </ul>

          <span>Copyright 2016, Original Trombones</span>

        </footer>

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