I mapped out the stimulus to the best of my ability, and found the assumption. My question is, why do we need to assume that there is vitamin d in the milk? Is there a reason why finding the assumption that older people will be able to absorb the calcium in a glass of milk would not be sufficient to find the correct answer?

I don't know if this is a correct way to reason through this question, so tell me if I'm wrong, but after reading the stimulus and identifying the conclusion, I looked for any variable that was not present in both the stimulus and conclusion. Those two variables happened to be money and happiness. Since sufficient assumption questions are asking us (in simple terms) to supply a missing premise to make an argument valid, I looked for any answer choice that talked about money and happiness. That is how I chose A. I eliminated answer choice E and D because they talk about wealth. That was too large of an assumption for me. B was silly, because we already know we cannot have happiness without health, so we cannot have money and happiness at the exclusion of health. C was wrong for me because we aren't given any reasons why we should value health, just that it is necessary for happiness. Any feedback is welcome.

I got the correct answer choice but haven't really seen anywhere (in a concise way), why answer choice A is incorrect. I eliminated A because, to me, it did not "bridge" the gap, or explain the phenomenon observed. It implied that the researchers data was biased, and could not be trusted to explain the difference between the two bird population's beak sizes. A tells me that there are birds with larger beaks, or at least with the same beak sizes as the ones in captivity that the scientists did not or could not catch which skewed their data in favor of the smaller size. Hoping that this is a correct line of thinking, and also to help people like me, who was looking for a concise explanation why A was wrong.

I got the correct answer through process of elimination. However, I don't really understand how to tell the difference between the times it is okay or even necessary to equate two words versus when it is absolutely not okay. I have gotten questions wrong because I equated two extremely similar words to mean essentially the same thing. Is there any way to tell or do we just have to get lucky?