Explanation
Correct answer: A.
Choice A is the best answer because it most logically completes the text’s discussion of microbial fuel cells (MFCs) and the effect of silver nanoparticles on power output. The text explains that in MFCs, electrons are transferred from the bacterial cytoplasm to an external electrode through a series of inefficient redox reactions, which typically prevent power output from exceeding 0.30 mW/cm². When researchers added silver nanoparticles to carbon paper covering the anode, power density more than doubled to 0.66 mW/cm². Critically, the text notes that metals such as silver exhibit high electrical conductivity. Given that the inefficiency of the redox reactions accounted for the limited power output of MFCs and that the addition of a highly conductive material dramatically increased that output, it is reasonable to hypothesize that the silver nanoparticles allowed electrons to bypass the redox reactions and transfer directly to the electrode.
Why the other choices are wrong
Choice B
Choice B is incorrect because the researchers’ hypothesis, as set up by the last sentence of the text, is specifically grounded in the high electrical conductivity of metals such as silver. Nothing in the text supports the idea that the silver nanoparticles function as catalysts for the redox reactions, nor does the text suggest that electrons are being conducted to the electrode independently of any role played by the silver nanoparticles. The text points to the conductivity of silver as the explanation for the increased power density, not to any catalytic effect of the nanoparticles on the existing redox reactions.
Choice C
Choice C is incorrect because the text never discusses the metabolic processes of the bacteria in the biofilm coating the electron-collecting anodes, nor is there any suggestion that the high conductivity of silver nanoparticles would interact with the bacteria’s metabolism in any way.
Choice D
Choice D is incorrect because the text never suggests that in the experiment, the biofilm on the electron-collecting anodes varied in density. Moreover, the hypothetical statement begun in the last sentence of the text concerns the high electrical conductivity of metals such as silver. Thus, proposing that redox reactions—which, as the text establishes, conduct electricity—accelerated independently of a conductive metallic material (the silver nanoparticles) wouldn’t logically complete that hypothesis.