All fish live in water. All sharks live in water.
Which one of the following can be logically concluded, based SOLELY on the statements above?
A. All sharks are fish.
(A) is incorrect even though it is true, because one cannot conclude this based SOLELY on the two statements above. Our outside knowledge about the animal kingdom tells us that sharks are fish, but the stimulus only tells us that they all live in water. It also tells us that all fish live in water, but does not create a category of "fish" that includes "sharks" as an example within that category. Neither statement excludes the possibility that other creatures besides fish (and sharks) live in water, and the two statements taken together, without more, do not exclude the possibility that sharks are not fish.
This question is an example of a syllogism, an attempt to define categories of things and examples of things within those categories and/or excluded from those categories. For example: All poodles are dogs. Fifi is a poodle. Therefore, Fifi is a dog. However, although all poodles are dogs, not all dogs are poodles. If all poodles are dogs, and Fifi is a dog, that doesn't mean that Fifi is a poodle.
(A) is sort of a trick answer, because as mentioned above we all know that sharks are fish and can't not be fish. Choosing (A) is an example of reaching the "right" conclusion for the wrong reasons, by substituting intuition for logic.
B. All fish are sharks.
(B) is incorrect and untrue, but if you answered (A), then (B) would have to be true as well. The stimulus states that both "fish" and "sharks" live in water, but doesn't tell us which is a category and which is an example within that category. If the fact that both sets of creatures live in water means that one is a subset of the other, without more, how do we know which is which?
C. If it lives in water, it must be a fish.
(C) is incorrect because the stimulus does not tell us that only fish live in water, it tells us that all fish live in water. Difference. If we know it is a fish, we know it lives in water because they all live in water, meaning that (contrapositive) there is no fish that does not live in water. But again, the stimulus does not exclude the possibility of creatures that are not fish who do live in water. If all we know is that it lives in water, it could be a fish, it is probably a fish, but it could be something else as well.
D. If it is not a fish, it does not live in water.
(D) is also incorrect because the stimulus does not tell us that only fish live in water. Once again, the stimulus does not exclude the possibility that creatures other than fish (and sharks) live in water. If non-fish can live in water, then the fact that a creature is not a fish does not by itself tell us whether the creature lives in water, let alone prove that it does not.
E. None of the above.
(E) is correct.
