Question: Biologist: Marine animals known as box jellyfish have eyes with well-formed lenses capable of producing sharp images that reveal fine detail. But the box jellyfish's retinas are too far forward to receive a clear image, so these jellyfish can receive only a blurry image that reveals prominent features of objects but not fine detail. This example shows that eyes are adapted only to an animal's needs rather than to some abstract sense of how a good eye would be designed.

The argument requires assuming which one of the following?

(A) Box jellyfish are the only kind of jellyfish with retinas that do not focus clearly.
(B) Box jellyfish have a need to detect prominent features of objects but not fine details.
(C) Box jellyfish would benefit from having retinas that allowed their eyes to focus more sharply.
(D) Box jellyfish developed from jellyfish whose retinas received clear images.
(E) Box jellyfish use vision as their main means of detecting prey.

Correct Answer: B
Explanation: Box jellyfish have a need to detect prominent features of objects but not fine details.- Correct.
We are told the jellyfish's eyes are designed in a way that allows them to see "prominent features of an object but not fine details", and are then told that this proves that the eyes are designed to fit an animal's needs. In other words, the argument equates the animal's needs with detecting prominent features, not fine details - exactly what (B) tells us.
Let’s examine the other available options.

Option A
Box jellyfish are the only kind of jellyfish with retinas that do not focus clearly.- Incorrect. The argument’s conclusion is general, describing how an animal’s eyes are adapted. Whether the box jellyfish is the only kind of jellyfish with retinas that do not focus clearly does not tell us anything about how an animal’s eyes are adapted to need. This answer choice is not required for the conclusion to be valid, nor is it even relevant to the conclusion.

Option C
Box jellyfish would benefit from having retinas that allowed their eyes to focus more sharply.- Incorrect. Since the biologist’s argument had to do with the jellyfish’s need rather than what would benefit the animal, this answer choice is irrelevant to the conclusion and is incorrect.

Option D
Box jellyfish developed from jellyfish whose retinas received clear images.- Incorrect. This answer choice could strengthen the conclusion, but is not required for it to be valid. First, it is not required, because we do not need to know the retinal capacity of the box jellyfish’s evolutionary predecessor to reach the conclusion that the animal’s current retinal structure emerged in response to the animal’s need. At the same time, it could strengthen the conclusion by indicating that although the animal at one time had a better constructed retina, that biological feature disappeared because it was no longer needed.

Option E
Box jellyfish use vision as their main means of detecting prey.- Incorrect. The answer choice could be tempting because it indicates that box jellyfish rely on their vision to detect prey. However, that reliance is not necessary for the conclusion regarding adaptation, because it does not tell us that in detecting their prey the box jellyfish need to see prominent features of objects but not fine detail.

“Biologist: Marine animals known as box jellyfish have eyes with well”- is a GMAT critical reasoning topic. This GMAT critical comes with five options and candidates need to choose the one which is correct. GMAT critical reasoning tests the logical and analytical skills of the candidates. To answer the question, a candidate can either find a piece of evidence that would weaken the argument or have logical flaws in the argument. Candidates get 65 minutes to answer 36 MCQ questions in the critical reasoning section of the GMAT.

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