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CAT Critical Reasoning Practice question with Solution 40

Nuclear fusion is a process whereby the nuclei of atoms are joined, or “fused,” and in which energy is released. One of the by-products of fusion is helium-4 gas. A recent fusion experiment was conducted using “heavy” water contained in a sealed flask. The flask was, in turn, contained in an air-filled chamber designed to eliminate extraneous vibration. After the experiment, a measurable amount of helium-4 gas was found in the air of the chamber. The experimenters cited this evidence in support of their conclusion that fusion ad been achieved.

Which one of the following, if true, would cast doubt on the experimenters’ conclusion?

[A]. Helium-4 was not the only gas found in the experiment chamber.
[B]. When fusion is achieved, it normally produces several by-products, including tritium and gamma rays.
[C]. The amount of helium-4 found in the chamber’s air did not exceed the amount of Helium-4 that is found in ordinary air.
[D]. Helium-4 gas rapidly breaks down, forming ordinary helium gas after a few hours.
[E]. Nuclear fusion reactions are characterized by the release of large amounts of heat.
Answer: C

We need to weaken the experimenters’ conclusion that fusion was achieved. The experimenters point to the presence of helium-4, a known by-product of fusion, in an airfilled chamber. So what? If the amount of helium-4 found in that air was no higher than that found in ordinary air, answer choice (C), then we would have reason to believe that the helium-4 gas found in the chamber was there before the experiment. If fusion had been achieved, then we would expect the level of helium-4 to be higher than the level found in the air in general. For this reason, answer choice (C) casts doubt on the experimenters’ conclusion.

(A) The last thing (A) does is cast doubt on the argument; we would almost expect to find other gases in the chamber if fusion was achieved, because we’re told that “one of the byproducts of fusion is helium-4 gas,” implying that there may be others.

(B) Same as (A): this is wholly consistent with the facts in the passage.

(D) The crux of the argument is that helium-4 was found in the chamber, which supposedly supports the notion that fusion was achieved. What happens to helium over time is irrelevant—the helium was found in the chamber, and the conclusion was based on that alone, end of story. Answer choice (D) therefore doesn’t affect this argument one bit.

(E) So what? We’re told nothing about “heat” in the stimulus, so there’s no way this little tidbit of information can do any damage to the argument. (We’re told that energy is released, and you may have associated that with heat, but this still doesn’t hurt the argument—if anything, it means only that (E) is consistent with the passage’s first sentence.)

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