The explanation for C, is regardless, awful. Holding these variables constant would make the scientific experiment stronger

Could this be explained more thoroughly by focusing instead on the fact that the stimuli says that the tissue biopsies were taken AFTER the surgeries? This helps me remedy the fact that the state of the patients/integrity of the study and surgery background is not important on the grounds that the biopsies were not taken during the surgery (which is what I assumed due to missing this detail in the stimuli). The video mentions this important detail, but I think the written explanation would be much more effective if the nature of the biopsies was emphasized as this is what makes this question actually tricky for those of us that tend to think beyond the scope of the question

This is confusing to me- with the correct answer alone, I suppose the explanation makes sense. However, I feel the explanation of why the wrong answers are wrong disregards the actual validity of said answers. How is strengthening the validity of the study, NOT strengthening the conclusion here? This is where my science degree hurts me. My immediate thought, without looking at the answers, was that bolstering the integrity of the study that provides the evidence for the conclusion, would strengthen the hypothesis. If the study has minimal flaws, then the correlation can be taken more seriously. 1- If the patients all had similar health stats, this ELIMINATES alternative hypotheses that these factors could be contributors to the abnormalities, it does not introduce it!! Just because it is not explicitly stated does not mean that the reader isn't questioning this. 2- with B and C- "The patients' throat surgery was not undertaken to treat abnormalities in their throat muscles," and, "people who have undergone throat surgery are no more likely to snore than people who have not undergone throat surgery."- Do these not strengthen the study? they both verify that the sample has no bias. When reading the stimuli, I immediately poked holes in the logic on the basis that those undergoing throat surgery are likely a biased sample, as people with throat issues are definitely more likely to snore. I just think this is a horrible question trained to trick people with a background in science