OG详解-OG5 文法 Q8

Choice D is the best answer. “As a result” logically signals that the information in this sentence—the vessel turning black—is a result of the heating technique discussed in the previous sentence.
Choice A is incorrect because “on the contrary” illogically signals that the information in this sentence directly opposes the heating technique in the previous sentence. Instead, the vessel turns black as a result of that technique. Choice B is incorrect because “for example” illogically signals that the information in this sentence is an example of the heating technique in the previous sentence. Instead, the vessel turns black as a result of that technique. Choice C is incorrect because “previously” illogically signals that the information in this sentence occurs earlier in a chronological series of events than does the heating technique discussed in the first two sentences. Instead, the vessel turns black as a result of that technique.