WordPress, Display parent pages only

<?php
function parent_pages(){
  $args = array(
        'post_type' => 'page',
        'numberposts' => -1
);
  $the_query = get_posts( $args );
       foreach ($the_query as $key) {
       if(!empty(get_children( $key->ID ))){
       echo '<li><a href="'.$key->guid.'">'.$key->post_title.'</a></li>';
   }
  }
}
   echo parent_pages();
?>

Display parent page as select option

<select style="background-color:black; color:white; padding:10px; " onchange='document.location.href=this.options[this.selectedIndex].value;'>
<option value="" selected="selected">Parent pages</option>
<?php
   function parent_pages(){
   $args = array('post_type' => 'page','numberposts' => -1);
   $the_query = get_posts( $args );

   foreach ($the_query as $key) {
    if(!empty(get_children( $key->ID ))){
    echo " <option value='". $key->guid. "'>". $key->post_title. "</option>";
  }
 }
}

  echo parent_pages();
?>
</select>