Challenge Line up Form Elements Responsively with Bootstrap

We line up the form elements the same way we do with others, using divs.

<div class="row">
<div class="col-xs-7"><input type="text" class="form-control" placeholder="cat photo URL" required></div>
<div class=col-xs-5><button type="submit" class="btn btn-primary"><i class="fa fa-paper-plane"></i> Submit</button></div>
</div>