A stingray without parasites is healthier than it would be if it had

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?

Discrepancy: the lack of parasites in stingrays is an indicator that the ecosystem in which the stingrays live is under environmental stress such as pollution.
We need an answer that tells us pollution is the reason behind lack of parasite.

(A) During part of their life cycles, the parasites of stingrays require as hosts shrimp or oysters, which are environmentally vulnerable organisms.
parasites' food=shrimp or oysters
But shrimp or oysters are environmentally vulnerable organisms.
So, pollution reduces 'parasites' food' , which in turn decreases parasites in stingrays.

(B) A stingray is a free-ranging predator that feeds on smaller organisms but has few predators itself.
Out of Scope.

(C) A parasite drains part of the vitality of its host by drawing nourishment from the host.
Out Of Scope.

(D) An ecosystem can be considered stressed if only a few species of very simple organisms can live there.
Irrelevant


(E) Since the life of parasites depends on that of their host, they need to live without killing their host or else to reproduce and infect other individuals before their own host dies.
Irrelevant.

A is our answer.

IMO D
(D) An ecosystem can be considered stressed if only a few species of very simple organisms can live there.
since parasites are a simple living organism , having a lack of such indicates that enviroment is not able to sustain a lot of parasites.

Thus D helps resolve the depenency
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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?


(A) During part of their life cycles, the parasites of stingrays require as hosts shrimp or oysters, which are environmentally vulnerable organisms.

(B) A stingray is a free-ranging predator that feeds on smaller organisms but has few predators itself.

(C) A parasite drains part of the vitality of its host by drawing nourishment from the host.

(D) An ecosystem can be considered stressed if only a few species of very simple organisms can live there.

(E) Since the life of parasites depends on that of their host, they need to live without killing their host or else to reproduce and infect other individuals before their own host dies.

SR healthy - when no parasites
not healthy - with parasites

no parasites - indicator of pollution, in which stingrays live

healthy stingrays live in environmental conditions which are unheathy in general such as pollution. Why?

if parasites require hosts that can not live in pollution, parasites can not live in such conditions either. And thus stingrays would be healthier there. i.e. to put into statements argument is trying to convey :-
lack of parasites -> indicator of pollution area - stingray lives in such areas
lack of parasites because they don't find shrimps or oysters needed for them to feed on. - thus stingray thrives in such areas

Answer - A

Parasites are bad for stingrays. But their absence is bad news for the ecosystem in which
the stingrays live. That’s the shift. How could parasites be good for the ecosystem even
though they are bad for some of the residents of that ecosystem?

If (A) is true, and stingray parasites depend on other environmentally vulnerable
organisms, then the presence of the parasite is an indicator of the presence of those other
organisms. The parasites are still bad news for the stingray, but they could be good news
for the ecosystem. If the parasites are around, then the species they depend on must be
present as well, which supports the notion that the system isn’t under stress. But if the
parasites are missing, then maybe those other species are missing, too, and this in turn
would support the notion that environmental stresses have taken a toll on vulnerable
organisms such as the shrimp and the oyster.

(B) is useless background information.

(C) explains why parasites are bad for stingrays (or any hosts), but we knew that already.

(D) is too broad: A general statement about what might constitute environmental stress
doesn’t help us to resolve the puzzle posed in the stimulus—why is something that’s bad
for an inhabitant of an environment a positive sign for the environment as a whole?

(E) explains why parasites cannot be too hostile to their hosts, but doesn’t connect
stingray parasites to the ecosystem in general.

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