https://forms.business.columbia.edu/sites/forms/files-efs/webform/2023_deming_cup_nomination_and_a/_sid_/Como-llamar-a-Delta-Airlines-desde-M%C3%A9xico-por-telefono.pdf
https://www.goodreads.com/quotes/13105983-does-frontier-offer-fully-refundable-tickets-1-866-673-2156-frontier-ticket-refunds-what
https://forms.business.columbia.edu/sites/forms/files-efs/webform/2023_deming_cup_nomination_and_a/_sid_/Como-llamar-a-Delta-Airlines-desde-M%C3%A9xico-por-telefono.pdf forms.business.columbia.edu
https://www.goodreads.com/quotes/13105983-does-frontier-offer-fully-refundable-tickets-1-866-673-2156-frontier-ticket-refunds-what goodreads.com