Just to be clear, this is an Assumption question and not a Justify the Conclusion question.
Remember that an assumption is simply an unstated premise- what must be true in order for the argument to be true. Also remember that arguments that use surveys rely upon the validity of those surveys for these types of questions. And so answer choices which strengthen or assume the survey's soundness, are often the correct answer choice.
This particular stimuli is arguing cause and effect. So it's important to keep in mind that the stimuli (which we accept as true) argues that fish oil consumption actually causes lowered incidence of heart disease. So what the surveyed participants who are eating fish oil are otherwise eating is not relevant to what is necessary for the argument to be true. Rather, what is necessary is eliminating a competing cause for the necessary effect. That effect being lower heart disease.
(D) is also arguably not just eliminating a competing cause, it is also supporting or proving the data used in the premise's survey which is another characteristic of a classic "right" answer choice in a question of this type.
I don't understand why B was eliminated because of the development of heart disease and diet which wasn't mentioned in the stimulus, but that that D is the correct answer and talks about exercise and cardiorespiratory health which isn't mentioned in the stimulus so I don't get why B which seems to resemble information in the stimulus the closet is wrong and why D is right when it includes new information not addressed in the stimulus.
To logically negate this answer choice simply remove the word "not" from the answer choice. doing so would tell us that the people who eat fish regularly help out their heart health in ways not related to fish. This would provide an alternate explanation for why they suffer fewer heart attacks.
I think the key here is the negation technique. Answer choice B, when negated, would read "The test subjects in the recent study who ate fish twice a week DID have a diet that was otherwise conducive to the development of heart disease." If that were true, though, it would make it even more astonishing that the test subjects were nearly 30% less likely to develop heart disease. In other words the negated form of answer choice B is not an attack on the argument, but in fact supports it strongly. Answer choice B, therefore, is incorrect.
Does that help?
Hi Clay! Thanks for answering. I hope you (or someone else) can clarify one thing. When I read answer choice B originally, I think I misread the sentence (my apologizes, English is not my first language). When the answer choice reads, "... conducive to the development of heart disease," does this mean that the diet is harmful to the heart (e.g. a diet of red meat)?
I read the sentence as a diet that helps positively augment cardio-respiratory health (like in answer choice D). Now, I think this was an incorrect reading.
It is indeed true that for the author to be correct that eating fish helped reduce heart disease, the author had to assume that nothing else was materially different about the middle-aged people in the study. Specifically, if those middle-aged people had done something else that would be likely to have a positive effect on their heart health, then the conclusion that is was the fish would be suspect. In short, the author must assume that this possible contrary evidence does not exist. The assumption here is a Defender; it is an assumption that a possible line of attack doesn't exist.
Answer choice (B) is not relevant because, in that answer, the diet assumed not to exist is one conducive to heart disease. In other words, (B) assumes that those eating fish in the study didn't have an otherwise bad, harmful diet. But if they otherwise had a bad, harmful diet, but also ate fish twice a week, and they had less incidence of heart disease, this certainly would not make me think the bad diet was responsible for less heart disease! (B) cuts the opposite way from the Defender you're looking for - the author must assume that the people in the study aren't doing other good things for their health (besides eating fish). (B) would be an assumption that they aren't doing bad things for their health. The author certainly does not need to assume that; in fact, if you negate (B), it actually makes the conclusion seem stronger - eating fish twice a week was able to improve heart health even with an otherwise heart-unhealthy diet! So the Assumption Negation technique should show why answer choice (B) is in fact opposite to what you want.
I was wondering if I could eliminate answer (B) because it doesn't provide information on both groups that are being compared in the stimulus. Answer (D) seems to do this, while answer (B) provides limited information. So the comparative element in the stimulus matches up with the comparison in answer (D). Is this an acceptable approach?