The information in the passage indicates that if human beings were given a drug that inhibits the action of serotonin...

I understand why A is correct but want some more clarification to show why D is incorrect, and why my initial thoughts about A were incorrect. When assessing A I understand the evidence that shows subjects would have a preference for Carbohydrates, but I don't understand how that is then extrapolated to mean they prefer it over protein-rich snacks. This is mentioned in the rat experiment, but it seems wrong to assume that is sufficient evidence to conclude humans would also react the same way. I agree with the statement that humans would prefer carbs and it "could be true" they prefer it over protein food but we don't know for sure. In response to D the wording of "probably" is what I focused on and believed it meant it "could be true" that blood tryptophan levels would increase since that is supported by the passage in lines 23-23 "so the proportion of tryptophan in the blood relative to the other amino acids increases when carbohydrates are consumed." I guess this also assumes that b/c you prefer carbs, you will eat them, leading to an increase in blood tryptophan levels. This is where I see the justification for A over D, but initially, it felt like we had to pick between two "could be true" answers. However, it still feels like both are equally valid, since A has to take one step to assume you would prefer carbs over protein, and D takes one step to assume you would eat carbs if you crave them. Can I get some clarification on those reasoning stems?