OG详解-OG9 文法 Q12

Choice B is the best answer. “By contrast” logically signals that the information in this sentence—that a firefly’s glow ceases when it stops drawing in oxygen—contrasts with the previous sentence’s discussion of the processes that cause a firefly to begin to glow. 
Choice A is incorrect because “for instance” illogically signals that the information in the sentence exemplifies the previous sentence’s discussion of how a firefly begins to glow. Instead, it contrasts with the previous sentence’s discussion. Choice C is incorrect because “specifically” illogically signals that the information in the sentence provides specific details elaborating on the previous sentence’s discussion of how a firefly begins to glow. Instead, it contrasts with the previous sentence’s discussion. Choice D is incorrect because “in conclusion” illogically signals that the information in the sentence sums up the previous sentence’s discussion of how a firefly begins to glow. Instead, it contrasts with the previous sentence’s discussion.