Explanation
Correct answer: B.
Choice B is the best answer because it most logically completes the text’s discussion of black swan familial structures. In this context, “elasticity” means flexibility or adaptability. The text provides examples of flexible family arrangements among black swans: successful male-female breeding pairs are known to foster other pairs’ cygnets, and while they are less common, male-male pairs also successfully rear cygnets. This context therefore emphasizes the flexibility or elasticity of black swan familial structures, showing how this trait contributes to the success of the species.
Why the other choices are wrong
Choice A
Choice A is incorrect. Although close family units may show “cohesion,” which refers to unity or harmony within a group, the text’s focus is on the variety and adaptability of black swan family structures. The examples given—fostering other pairs’ cygnets and forming male-male as well as female-male pairs—highlight flexibility rather than cohesion, emphasizing family structures that vary from the model in which a breeding pair raises its own young.
Choice C
Choice C is incorrect. While “affection,” or fondness, might be present in black swan relationships, the text doesn’t focus on emotional bonds but rather on the structural flexibility of black swan familial arrangements. The examples provided demonstrate adaptable breeding and rearing practices rather than affectionate behaviors.
Choice D
Choice D is incorrect because “reciprocity” would suggest mutual exchange or cooperation between parties. Although the text mentions the fostering of cygnets by different successful breeding pairs, it doesn’t establish that this is based on reciprocal exchanges. The focus is instead on the flexibility of family structures and the effectiveness of these varied structures in rearing young.