protected function submit_button(question_attempt $qa, question_display_options $options) {
        if (!$qa->get_state()->is_active()) {
            return '';
        }
        $attributes = array(
            'type' => 'submit',
            'id' => $qa->get_behaviour_field_name('submit'),
            'name' => $qa->get_behaviour_field_name('submit'),
            'value' => get_string('check', 'question'),
            'class' => 'submit btn btn-secondary',
        );
        if ($options->readonly) {
            $attributes['disabled'] = 'disabled';
        }
        $output = html_writer::empty_tag('input', $attributes);
        if (!$options->readonly) {
            $this->page->requires->js_init_call('M.core_question_engine.init_submit_button',
                    array($attributes['id'], $qa->get_slot()));
        }
        return $output;
    }