how to use bootstrap classes in wtforms

{% extends "bootstrap/base.html" %}
{% import "bootstrap/wtf.html" as wtf %}
{% from "_formhelpers.html" import render_field %}
{% block content %}
  <div class="container">
  <h3>Please answer this very important essay</h3>
  <p>If you don't it'll go on your permanent record!.</p>
<hr>
<form action="" method="post" class="form" role="form">
  {{ form.csrf_token() }}
 <d1>
   {{ render_field(form.essay_question, class='form-control', placeholder='Write down your thoughts here...') }}
   {{ render_field(form.email_addr, class='form-control', placeholder='Enter email') }}
 </d1>
 <p><input type=submit class='btn btn-warning btn-block'>
</form>
<hr>
<p>Copyright 2018 <a href="https://john.soban.ski">Freshlex, LLC</a></p>
</div>
{% endblock %}