Aspect Ratios(21by9)
<div class="embed-responsive embed-responsive-21by9">
<iframe class="embed-responsive-item" src="https://www.youtube.com/embed/h4pxLHG0Wzs" allowfullscreen></iframe>
</div>Aspect Ratios(16by9)
<div class="embed-responsive embed-responsive-16by9">
<iframe class="embed-responsive-item" src="https://www.youtube.com/embed/bk7McNUjWgw" allowfullscreen></iframe>
</div>Aspect Ratios(4by3)
<div class="embed-responsive embed-responsive-4by3">
<iframe class="embed-responsive-item" src="https://www.youtube.com/embed/OD6cQ2dh7Mo" allowfullscreen></iframe>
</div>Aspect Ratios(1by1)
<div class="embed-responsive embed-responsive-1by1">
<iframe class="embed-responsive-item" src="https://www.youtube.com/embed/F2ethtmiFyQ" allowfullscreen></iframe>
</div>