OG详解-OG8 文法 Q20

Choice C is the best answer. The convention being tested is the punctuation of a supplementary element within a sentence. The dash after “Springs” pairs with the dash after “earth” to separate the supplementary element “in this case, the porous rocks of the hills around Hot Springs” from the rest of the sentence. 
Choice A is incorrect because it fails to use appropriate punctuation to separate the supplementary element from the rest of the sentence. Choice B is incorrect because a colon can’t be paired with a dash in thisway to separate the supplementary element from the rest of the sentence. Choice D is incorrect because a comma can’t be paired with a dash in this way to separate the supplementary element from the rest of the sentence.