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<select name="archive-dropdown" onchange="document.location.href=this.options[this.selectedIndex].value;">
    <option value=""><?php esc_attr( _e( 'Select Month', 'textdomain' ) ); ?></option>
    <?php wp_get_archives( array( 'type' => 'monthly', 'format' => 'option', 'show_post_count' => 1 ) ); ?>
</select>