The correct answer choice is (C)
The argumentation in this stimulus is fairly straightforward:
- Premise: Fusion creates helium-4 as one of its by-products.
Premise: After the heavy water experiment, they found traces of helium-4.
Conclusion: Fusion was achieved (this was the conclusion of the “experimenters”).
The stimulus is followed by a Weaken question, so the correct answer choice will provide some basis for questioning the experimenters’ conclusion.
Answer choice (C) is the correct answer choice. If the amount of helium-4 was what would have been expected under normal circumstances, then the presence of helium-4 in the experiment chamber certainly doesn’t prove that fusion took place.
Answer choices (A), (B), and (E) are incorrect for related reasons. There is no way to assess, based on the limited information provided in the stimulus, the relevance of information such as the absence of other gases in the experiment chamber (answer choice A), or the relevance of normal by-products of fusion such as tritium, gamma rays (answer choice B), or heat (answer choice E). The author tells us only that helium-4 is “one of the by-products” of fusion. Answer choice (D) is incorrect for a different reason; if helium-4 rapidly breaks down, then its presence in the experiment chamber strengthens the experimenters’ conclusion.