how can i show only the parents in owl-carousel?

I have this code that shows all the posts in my post type (parents and children). the php is:

<div class="owl-carousel owl-theme">             <?php             $  c=0;             $  q2=new WP_Query(array('post_type'=> array( 'post', 'book' ),'post_status'=> 'publish','orderby'=>'modified','order'=>'desc'));             while($  q2->have_posts()) {                 $  c++;                 $  q2->the_post();                 ?>                 <li class="item">                     <a href="<?php the_permalink(); ?>">                         <?php the_post_thumbnail(); ?>                         <div class="metaitem">                             <ul>                                 <li><?php the_title(); ?></li>                                 <li><?php the_excerpt(); ?></li>                             </ul>                         </div>                     </a>                 </li>                 <?php             }             wp_reset_postdata();             ?>         </div> 

now what array i can use to show only the parents and don’t show the children? i have used 'parent' => 0 but it didn’t work!