bootstrap carousel arrows not showing code example

Example: bootstrap carousel arrows not showing

<a class="carousel-control-prev" href="#projectCarousel" role="button" data-slide="prev">
        <span class="carousel-control-prev-icon"></span>
</a>
<a class="carousel-control-next" href="#projectCarousel" role="button" data-slide="next">
        <span class="carousel-control-next-icon"></span>
</a>