Explanation
Correct answer: B.
Choice B is the best answer because it most logically completes the text’s discussion of Smith and colleagues’ investigation of the evolution and biological role of the appendix. The text indicates that the team found several instances of the appendix emerging and not disappearing in the lineages of various mammal species the team examined. Furthermore, the text states that species that possess an appendix also tend to have relatively high amounts of lymphoid tissue—a type of tissue that supports immune system function. Taken together, these details strongly support the hypothesis that the appendix has persisted in some species because it has a function that contributes to effective immune responses in those species.
Why the other choices are wrong
Choice A
Choice A is incorrect because the text doesn’t address any nonmammalian species.
Choice C
Choice C is incorrect because the text doesn’t make predictions about the evolutionary future of the species Smith and colleagues examined, and although the implication of the text is that the appendix likely does serve a function for the immune system, nothing in the text indicates that the appendix will become more widespread in the future.
Choice D
Choice D is incorrect. Although the text does suggest an association between having an appendix and relatively high concentrations of lymphoid tissue, it doesn’t claim that the appendix causes the tissue to grow, nor does it address the relative production of the tissue at different periods of time.