新GRE argument

新GRE argument
新G题库中argument第二题
2.The following appeared as part of a letter to the editor of a scientific journal.
"A recent study of eighteen rhesus monkeys provides clues as to the effects of birth order on an individual's levels of stimulation.The study showed that in stimulating situations (such as an encounter with an unfamiliar monkey),firstborn infant monkeys produce up to twice as much of the hormone cortisol,which primes the body for increased activity levels,as do their younger siblings.Firstborn humans also produce relatively high levels of cortisol in stimulating situations (such as the return of a parent after an absence).The study also found that during pregnancy,first-time mother monkeys had higher levels of cortisol than did those who had had several offspring."
Write a response in which you discuss one or more alternative explanations that could rival the proposed explanation and explain how your explanation(s) can plausibly account for the facts presented in the argument.
请问如何破题?求各路高手相助~