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.
Your approach to answers A and B looks good to me, andriana.caban - the author never said H4 was the only byproduct, so other gasses could be fine. We don't know if tritium or gamma rays happened, so maybe they did - also fine.
For answer E, I'm not sure what you're getting at there, but it's just like answer B - there was supposed to be a lot of heat, and we don't know from the stimulus that there wasn't, so perhaps there was. Think of heat as being just like any other byproduct - it was supposed to be there. If we knew it was not there, that would weaken, but if we are left wondering, then no problem.
As to answer D, by the way, I would not say that it proves the argument. All it tells us is that the H4 we found must have gotten there recently, and that doesn't prove that fusion happened. Maybe H4 is also a byproduct of, say, fission, and fission happened instead of fusion? Maybe it's a byproduct of whatever the experimenters had for lunch that day? Still, answer D doesn't weaken the argument, and it does strengthen at least a little by eliminating the possibility that it was just there in the air all along.
I had trouble understanding this one. I chose B. When I look at C again, I look in the stimulus when it says when fusion is achieved their is a by product of measurable helium 4 gas. If the helium 4 is not measurable to the same extent of the by product of fusion, it weakens the conclusion. Thats how I look at answer C now. Does that sound about right?
whardy21 wrote:I had trouble understanding this one. I chose B. When I look at C again, I look in the stimulus when it says when fusion is achieved their is a by product of measurable helium 4 gas. If the helium 4 is not measurable to the same extent of the by product of fusion, it weakens the conclusion. Thats how I look at answer C now. Does that sound about right?
Hmm, a bit, but there's more to it if I'm following what you said correctly. Imagine for a moment that in regular air there is already a bit of helium-4, call it 10 atoms worth. No matter what, it's always there. Now, it's also a product of fusion, but you get a lot more helium-4 when there's fusion, say 50 atoms worth (this part isn't necessary, but it helps make this example clearer). So, these scientists do their experiment, and then measure, and there's helium-4 there. Should they immediately claim that there was fusion? No, they need to measure how much was there. If it's only 10 atoms worth, all we have is what is normally in the air, and that would certainly make us question whether there was fusion. This is what (C) is saying, and is why (C) weakens the argument.
By the way, if we had more atoms of helium-4 than just 10, not we'd start to think something else had occurred.
Does that help at all? Please let me know. Thanks!