Why is there no vertical space between Bootstrap 4 rows

问题

I am trying to migrate from Bootstrap 3 to Bootstrap 4. Bootstrap 3 had vertical space between rows by default, but this is not the case in Bootstrap 4. Can someone please explain why? Or am I missing something in my HTML?

<div class="container-fluid">
    <div class="row">
        <div class="col-9">
            <div class="row">
                <div class="col">
                    <div class="card">
                        <div class="card-header">
                            Card01
                        </div>
                        <div class="card-body">
                            Card01
                        </div>
                    </div>
                </div>
            </div>
            <div class="row">
                <div class="col">
                    <div class="card">
                        <div class="card-header">
                            Card02
                        </div>
                        <div class="card-body">
                            Card02
                        </div>
                    </div>
                </div>
            </div>
        </div>
    </div>
</div>

回答1:

Rows no longer have margin-bottom in Bootstrap 4. Use the new spacing utils (mb-4).

See https://stackoverflow.com/a/42960338/171456.

来源：https://stackoverflow.com/questions/46452586/why-is-there-no-vertical-space-between-bootstrap-4-rows