Dynamically create active class within loop

index.php

<div class="carousel-inner" role="listbox">

    <?php
        $c = 0;
        $class = '';
        query_posts('category_name=slider&showposts=3');
        if ( have_posts() ) : while ( have_posts() ) : the_post();
            $c++;

            $thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'thumbnail' );
            $url = $thumb['0'];

            if ( $c == 1 ) $class .= ' active';
                ?>
                 <div class="item <?php echo $class; ?>">
                     <img src="<?php echo $url; ?>" class="img-responsive" alt="...">
                     <div class="carousel-caption">
                          <h2><?php the_content() ?></h2>
                     </div>
                 </div>
                 <?php
        endwhile;endif;
        wp_reset_query();
    ?>

</div>