Collapsing in Bootstrap 3?

Question

Bootstrap

Hey!
In Bootstrap 3, a beginner, there was a problem with the collapse of the parent of the elements, when laying out on pure css we give the parent a class .clearfixand everything is fine, how to be in Bootstrap? I think I missed something? If it helps, there is neither ::before nor ::after in the inspector.

<div class="container-fluid">
    <div class="row">
      <div class="col-md-3 col-sm-3 col-xs-12"></div>
      <div class="col-md-3 col-sm-3 col-xs-12"></div>
      <div class="col-md-3 col-sm-3 col-xs-12"></div>
      <div class="col-md-3 col-sm-3 col-xs-12"></div>
    </div>
  </div>

.row div {
  background-color: #999;
  height: 200px;
}