OG详解-OG7 文法 Q19

Choice C is the best answer. The convention being tested is pronoun-antecedent agreement. The singular pronoun “it” agrees in number with the singular antecedent “industry” and clearly indicates that the industry consists of just one or two suppliers per municipality. 
Choice A is incorrect. The plural pronoun “these” neither agrees in number with the singular antecedent “industry” nor clearly indicates that the industry—not another plural noun in the sentence, such as “start-up costs” or “barriers”— consists of just one or two suppliers per municipality. Choice B is incorrect because the plural pronoun “they” doesn’t agree in number with the singular antecedent “industry.” Choice D is incorrect because the singular pronoun “this” is ambiguous in this context; the resulting sentence leaves unclear what consists of just one or two suppliers per municipality.