A stingray without parasites is healthier than it would be if it had parasites. Nevertheless, the lack of parasites in stingrays is an indicator that the ecosystem in which the stingrays live is under environmental stress such as pollution. Which one of the following, if true, most helps to reconcile the discrepancy indicated above? ['During part of their life cycles, the parasites of stingrays require as hosts shrimp or oysters, which are environmentally vulnerable organisms.', 'An ecosystem can be considered stressed if only a few species of very simple organisms can live there.', 'Since the life of parasites depends on that of their host, they need to live without killing their host or else not reproduce and infect other individuals before their own host dies.', 'A parasite drains part of the vitality of its host by drawing nourishment from the host.']