PrepTest 103, Section 3, Question 18
A stingray without parasites is healthier than it(31 more words not shown)
Which one of the following,(10 more words not shown)
- ADuring part of(18 more words not shown)
- BA stingray is(13 more words not shown)
- CA parasite drains(13 more words not shown)
- DAn ecosystem can(15 more words not shown)
- ESince the life(29 more words not shown)
Reading the argument
Good for one stingray doesn't automatically mean good for the whole ecosystem.
The stingray is healthier without parasites than that same stingray would be with them. I can accept that. But the passage also says their absence is a sign of environmental stress. Those statements concern different things: the effect on an individual animal and what is happening across the surrounding ecosystem.
My first guess is that parasites might have some ecological role despite harming the stingrays. Think of an animal people dislike, such as a snake, that might keep another population in check. Losing it could upset a balance even if some individual animals benefited. That is a possible kind of explanation, not a fact we have been given about these parasites.
What we’re looking for
We're asked to reconcile the apparent discrepancy. Why would the absence of something that harms a stingray be a sign that its environment is in trouble? I'll look for the missing connection between parasites and the larger ecosystem, while keeping both stated facts intact.
Working the answers
- ACorrect
This explains the connection differently from my initial guess. The parasites don't have to perform a beneficial role. Their absence can reveal a problem with other organisms they need.
During part of their life cycle, they require shrimp or oysters as hosts, and those hosts are environmentally vulnerable. If pollution harms the shrimp or oysters, the parasites may be unable to complete their life cycle. The stingrays could then lack parasites because the earlier hosts are in trouble.
So the absence of parasites can be a warning sign, not the cause of the ecosystem's problems. The individual stingray benefits from having no parasites, while that very absence can signal damage elsewhere in the system. We don't need to prove that every missing parasite establishes a missing oyster; the proposed link makes sense of why the absence serves as an indicator.
- B
This describes the stingray's place among predators and prey, but it gives no connection between missing parasites and environmental stress. Knowing that a stingray has few predators doesn't explain why its lack of parasites would tell us something about the wider ecosystem.
- C
This explains why a parasite can make its host less healthy. That fits the first statement, which wasn't the puzzling part by itself. We still need to understand why the absence of that harm would be a sign of trouble in the environment.
- D
Losing the stingrays' parasites doesn't establish that only a few simple species can survive there. The area might still contain many kinds of fish, birds, and other organisms. D gives a possible sign of stress, but it doesn't show that the particular sign in our passage meets that description.
- E
This describes how parasites can survive and reproduce despite depending on hosts. It allows either keeping the host alive or spreading before the host dies. Neither possibility explains why parasites being absent from stingrays would indicate environmental stress. The missing link is to the wider ecosystem, which A supplies through the vulnerable earlier hosts.
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