OG详解-OG4 文法 Q19

Choice D is the best answer. The convention being tested is the punctuation of a supplementary element within a sentence. The comma after “(13C)” pairs with the comma after “isotope” to separate the supplementary element “carbon-13 (13C)” from the rest of the sentence. This supplementary element defines the “rare carbon isotope,” and the pair of commas indicates that this element could be removed without affecting the grammatical coherence of the sentence.
Choice A is incorrect because it fails to use appropriate punctuation to separate the supplementary element “carbon-13 (13C)” from the rest of the sentence. Choice B is incorrect because it fails to use appropriate punctuation to separate the supplementary element “carbon-13 (13C)” from the rest of the sentence. Choice C is incorrect because it fails to use appropriate punctuation to separate the supplementary element “carbon-13 (13C)” from the rest of the sentence. The comma after “carbon-13” isn’t necessary because the parentheses around “13C” already separate this element from the rest of the sentence.